You can download WhatsApp Beta 2.21.10 from the following sources:
Beta 3310 focuses heavily on backend fixes: faster database migration when transferring chats to a new phone, reduced RAM usage on older Android devices, and a fix for the infamous "stuck at 'Initializing'" backup restore bug. whatsapp beta 3310 download link
This is the safest method, though you may not get the exact "3310" build number immediately, as beta slots fill up quickly. You can download WhatsApp Beta 2