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You could either look for it online in the core documentation linked above or use git to find the commit that introduced the deprecation as that commit also added the file.
Most files that cover deprecations also document migration strategies for affected code.
As usual there might be additional things that are good ideas during the upgrade process of such a project which I do not cover.
Please don't hesitate to contact me on Slack or via Email so I can incorporate them here for everybody.
The documentation file for the example used here can be found under Deprecation: #69057 - Deprecate Icon Utility and move methods into Icon Factory. Your ultimate goal is to have no entries in the deprecation log triggered by your own extensions.
You have no direct influence on the code of the TER extensions that may produce deprecation log entries.
If your installation only uses a few extensions and a single page tree with first and foremost mere content - then you are probably fine and do not really need to care too much about everything I am going to say in this post.
But if you happen to work on a rather big installation that has complex applications going on and is somewhat crucial to your client (as most websites are) you might find some of this advice helpful.
Removed or changed APIs Chasing errors after an upgrade can be very frustrating, especially if the API changed and your code breaks because of it.
To prevent such a scenario you should do some research before the upgrade.
Upgrading from 7 LTS to 8 LTS will be significantly less painful than touching a really old project might be.