Rainloop won’t be back.
You should see a “Simple” section, with “rainloop.user”.
Just click on “rainloop.user”, you will get an easy-style window like you had with Rainloop, then use “+ Add a filter” button:
Hint: if you accidentally switched to source editing and get a script editing windows, you can use the
button at bottom right to switch back to easy WYSIWYG mode 
Don’t forget to save:
If using Thunderbird from a PC, you can also use the excellent Sieve extension.