This means removing ABBC3 will not break your old posts, for the most part.
All of ABBC3's BBCodes will still be present as regular custom BBCodes. You can edit and/or delete them if you choose by visiting the Custom BBCodes section of the ACP.
There are only a few special BBCodes, however, that will not work as expected if ABBC3 is removed:
[BBvideo]BBvideo (will revert to a url link)
[fade]Fade text (will no longer fade in/out)
[hidden]Hide content from unregistered guests (content will be hidden to all users)
[spoil]Spoiler message (Spoiler content will be hidden)
[youtube]Youtube legacy BBCode (will revert to a url link)
You can re-install ABBC3 at any time and all its BBCodes will be restored (even if you edited or deleted some of them).