A few things:
1 I don't like the term stallers so I'll be referering to them as walls or defensive pivots or pivots or foggers or rockers or other things.
2 seriously just as smogon forums or the ou chatroom or somethin....
3 I play ou a lot so I'll do the best I can, but always check more reliable sources or just play for yourself.
Essentially, defensive pokemon currently are and always will play the role of doing various things on a wide variety of teamstyles. Asking whether they are good or not is relatively odd question, as the will always have some amount of viability. In terms of stall as a whole, compared to other gens, isn't doing great,but as a whole isn't doing bad either. Balance and BO teams are thriving and are most of the main teamstyles currently. Future sight did make toxapex drop for some time, but it has risen up a bit, and now dropping a tiny bit via slowing galar coming back a bit. There is a check or counter for every month, and nothing currently appears to be overly unbalanced. In terms of offensive threats, tapu lele, weaville, urshifu-r, and dragapult appear to be the main threats, while other pokemon such as blacheplon, kyurem, and kartana work well but not as large of a presence. In terms of however it was when you left,which I don't know when, as always, the meta is growing and changing and evolving. As it will for a long time, defensive pivots and walls still exist to provide a backbone for many teams. While some have dropped in usage, and unlikely candidates have risen, it stays in a balance still.
Pokemon like corviknight, landorous, toxapex, and others still remain at the forefront of the metagame.
In terms of some buffs and debuffs to the meta and stall as a whole.
Heavy duty boots comes and a double edged sword. In older gens, hazards on stall teams were a common sight, allowing for easier chip damage. Now, boots allows for avoidance of hazards unless they are knocked. This allows stall teams to be not worn down as fast, but also means they have a harder time wearing down opponents. Teleport provides safe switches, and also serves as a double edged sword. It means that while it can pivots into other teammates, it also means that if too many boots are knocked of or there is future sight, it can become threatening. Speaking of future sight, while it's usage has fallen by a substantial amount, it still remains as a way to get a ko every time you can set it up with urshifu.
Hazards, hazards removal, and playing around hazards
As mentioned above, hazards have taken a blow because of boots. This does not mean however, ghat defog is not nessicary of fatter teams. Knocking items is at an all time high in pokemon currently, so keeping boots isn't the easiest job. Because of boots as well, some pokemon do not need so much hazard remov support, such as volcarona, but other pokemon normally not weak to it are at risk of getting chipped greatly with no way to remove iI. So, hazards are still prominent as there is enough of a mind game in the teambuilder on what to do with hazards. Hazards provide valuable chip which pokemon like dragapult and weaville greatly enjoy. However, removing them is not a large problem either. Corviknight beat basically every rocker, and defogs very consistently as it is given many opportunities to come in. While hazards have taken a blow with boots, they still exist as a viable part of the OU meta.
Specific defensive mons viability and why,as well as good ones and not so good ones
So we all know that some mons work better than others. But, each mon can do a different thing than another, therefore allowing multiple mons to be viable on a given team or teamstyle. The viability of a mon depends often on what it can do and how well it can fit on a team. Pokemon like corviknight, landorous, and toxapex provide utility in various ways, and can be fitted on more teams than, say, a moltres(thought that dropped of other reasons). I'm not going to go over many of these such defensive pokemon as there is a whole viability rankings page on smogon for that, but I will go over a few. Corvimnight: it's amazing typing, stats, Ability, and usable movepool allows it to be a solid pick in the meta. It can defog, pivot out, and switchin/ scout many different pokemons sets. This allows for the #2 spot in usage in the last month.
Toxapex: Toxapex finds itself in a spot once again where it can be extremely helpful against main main threats like urshifu, weaville, and dragapult. Being able to knock, set up tspike, scald, and haze all with reliable recovery, a very good ability, and very solid stats, lead it to being a very good pokemon defensively.
Lando: why do I need to explain this bruh. Lando can knock, toxic, pivot out, set up rocks, defog, do strong damage, possibly even sweep, and provide good utility in Intimidate. Sp Def lando also allows for heatran switchins and free rocks.
Is this info necessary to play OU? No. Is it useful to know? Yes. Even with the most detailed explanations, you still need to find out for yourself. So I encourage you to just go out, have some fun and play some mons. But as for defensive pokemon specifically, some work better than others, but are they by no means whatsoever going down in usage as a whole. The metagame has once again found itself in a balance where every decent pokemon has its place, and defensive pokemon will always have a niche to bringing in weaker pokemon for free or taking hits for the team. As I've mentioned multiple times now, the easiest way is to just play the game and find out for yourself. Some mons might pose more or less of a threat just based on the teams you use or how you play. Defensive pokemon as a whole though, offer much in terms of hazards and hazard removal, pivots, chipping others down, providing a way to force others out, and much much more on almost all of the playstyles.
Also yes I'm not dead, but Mr stik(idk if anyone even knows what I'm talking about at this point lol) wanted me to answer this