Why would chat apps need to support old clients? If facebook pushes out an update almost every user will have it on day one and every user will 1 month on. They can just show an error banner to users who have automatic updates turned on.
It’s why apps like desktop Chrome have extremely aggressive automatic-update schemes which are near-impossible to disable if you’re ever connected to the internet.
iOS users upgrade pretty quick, but Android users don't. There are lots of phones with not very much storage, so upgrading is painful for users and they turn off auto-updates and don't update frequently.
And when you add on end of life platforms that have enough users you don't want to cut off, but not enough users that you want to do active development, or the platform owner won't sign packages anymore, you have to support some clients for a long time.