Deprecated messages are hints for the developers to change their program code. Due to the fact that the Nextcloud contains a lot of code and the number of developers is limited, it will most likely last a while to get rid of all those messages. So my recommendation is, to keep your installation up-to-date (Nextcloud 20.0.2 is already out!) and wait until all required code changes have been applied.
If you want to get rid of the messages earlier, you’re free to send push-requests in the relevant GitHub repositories and help to get these issues fixed