From Gen 4 onwards, base form Charizard would most likely not have been competitively viable in OU even if it was neutral to Stealth Rock for the following reasons. There are many Pokemon with similar stat lines and movepools to Charizard that are also bad in OU such as Porygon-Z and Kingdra despite being neutral to stealth rock, who suffer from the same issues listed below.
1. General power creep: The overall power level from Gen 4 onwards is much higher and Charizard doesn't really have the stats to keep up in this environment. This gets worse as the generations increase.
2. Competition from other fire type offensive Pokemon: Notable examples in OU are Infernape, Heatran, and Volcarona, Victini and Volcanion. These Pokemon all have some combination of better offensive or defensive typings, better movepools or abilities and better matchups against common Pokemon. Even in lower tiers like UU, it suffers stiff competition from Moltres, Arcanine, Rotom-Heat and others.
3. Bad matchups against common defensive Pokemon: If you take a look at Gen 4 OU's list of commonly used Pokemon, there are a lot of Pokemon like Latias, Clefable, Tyranitar etc, that completely stonewall Charizard. It doenst really have the needed offensive stats to break through the kind of defensive mons you run into in OU. Gen 5 onwards is a similar story. It also doesnt really have a very good defensive typing, being weak to rock ,water and electric, very commonly used coverage types for most offensive Pokemon.
The Mega forms of Charizard are viable in Gens 6-7 OU even with their stealth rock weakness, and they would of course be much better without it. They are held back quite a bit by their weakness because any team that uses them has to either be very fast paced or have good hazard removal methods.
There were a few gimmicky Sun teams in Gen 8 OU that used Choice Specs Charizard as an absurdly strong wallbreaker in the Sun. Those teams had a ton of issues besides Charizard being bad, so I dont think they would have been any less gimmicky with a stealth rock neutral Charizard.
What tier would Charizard end up in if it was neutral to Stealth Rock?
That's impossible to say with accuracy, but it would still face stiff competition from other fire types like Moltres, Arcanine, Rotom-Heat etc. So it propably wouldn't rise much in tiers. In gen 4 and gen 5 it would propably get banned from NU but not be able to get enough usage to get into UU or RU, so it would just be stuck in NUBL limbo. In gen 6 and gen 7 onwards the mega forms would be much better and may even end up getting banned from OU, but it's hard to predict because the format is so complex. The base form in gen 6 and gen 7 might also rise a tier or two. Charizard has a ton of problems other than 4x rock weakness that would still hold it back.
Note: This answer is for 6v6 Smogon singles for Gens 4-9 of OU, since no format was specified and that's what I'm most familiar with. Most of what I've said here does still apply to VGC and doubles though.
