at&t's notice did not use your "dropping service" verbiage. i currently have an at&t branded blackberry q10 (which i love) but apparently after january 2022 my blackberry q10 will no longer work on at&t's network. here is a copy/paste of the notice i received (and inside friend verified with his sources is correct)
3G Sunset - Single Line Customers
We're upgrading our network. You'll need a new phone by Feb. 2022.
Go online to www.att.com/3gmigration
to upgrade and replace your phone with a newer one.