Just answered another OP on this subreddit, but in short yes. Im typing this from a note 9 2018, not a lag in sight runs 8 hours for the battery. S21ultra from 2021? 888 snapdragon, pretty modern and versatile camera setup not s23 ultra but still very good. Still gorgeous display, will still get updates for a while and will last a very long time. For its price now highly recommend!

If u dont care heat, drain battery while standby (exynos version), it will be fine in future

If you already have it, then it’s fine. If you are looking to buy it, then it isn’t the best choice unless you can get an awesome deal on it. The snapdragon 888, 8 gen 1 and the equivalent exynos SoCs aren’t very good especially with the 8 gen 2 out. I’d spring for at least an 8+ gen 1.