Level is not a factor in the capture formula, nor has it ever been. HP is a factor, but both current and maximum HP are, so that they essentially cancel each other out - what really matters is the fraction of HP the Pokémon has left. This actually means Pokémon can be ever so slightly easier to catch when they're higher-leveled, if you're using False Swipe - a Pokémon with 1/20 HP is at 5%, but a Pokémon with 1/200 HP is at 0.5%.
The Level Ball and Nest Ball do take level into account when determining their ball bonus, but no other balls do.
Your perception that lower-leveled Pokémon have been easier to capture before is likely simply because later-game Pokémon, which are found at higher levels in the wild, broadly tend to have lower catch rates.
If you're curious about exactly what factors affect the capture formula and how, my site has a detailed article on it.