This actually does make sense because some plants don't like to be continuously uprooted and will die. The most you can do is a 90% change without hurting the plants at the bottom. If you do a 100% change you will also ruin the cycle, all the natural good bacteria will be gone and you would have to start from scratch; and would cause more problems making it unstable. If you cycle the tank it will stay healthy and if you do 50% changes every week you will get rid of the ammonia. Just test your water a lot in case problems arise.
I said for non-rooting plants.....
the concept of you can't do 100% water changes because you will 'kill' the good bacteria is a myth. even in a cycled tank, 99% of the bacteria is your filter media and even if you do a 100% water change, you rarely will cause the cycle to crash. you are far more likely to crash your cycle if you mess with filter media. If you tank has insanely high nitrates or something, than its better to do a 100% water change than to risk poisoning your fish.
without a filter, there isn't really enough bacteria to make a difference. plants do not cycle
your tank, they can't turn ammonia into nitrate, only bacteria can do that. the only thing plants can do is they can absorb ammonia which reduces the amount of water changes...
So right now:
-No filter but heated
-A few plants
-50% water changes weekly
-Do I need to vacuum? Some said don't because the plants "need" the poop..
plants don't need the 'poop', the poop decays to become ammonia which the plant can use but the plants cannot absorb all of the ammonia which is why you do water changes. fish create ammonia continuously through their gills too so there is really no reason to leave the poop there. you risk having too much decay and ammonia poisoning. I don't recommend gravel, they are such a pain to clean and end up hoarding lots of poop/debris which decay, but if you really want gravel, you will need to vacuum.
imo, you should try to get an ammonia test kit. its the best way to find out when/how much water you need to change. sometimes all you need is 25% wc a week if you got lots of plants, sometimes you might have to do two 50% wc if your plants don't use ammonia very well.