"TalkBack (...) port our changes (other than Play services vision library removal since it now provides useful OCR functionality we need to replace rather than removing)" absolutely! If we ever (hopefully) find FOSS-licensed OCR…
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There are probably some options but the main issue is that they are probably nowhere near as good, likely also have potential security issues and it would be difficult to hook them up in the same way since they won't be providing anything close to the same high-level API.