I have no idea if this is intended for some reason or just a bug and I'm not 100% sure when it disappeared because I very recently got a new phone and it's possible my old phone had a much older app version. But I have noticed it in my version 10. 51.1
I used to have a strikethrough and a highlight option in the app on the same "task bar" (circled in included screenshot) that has bold/italic/underscore etc. But now it isn't there any more. My current options are some AI thing, bold, italic, underscore, font, checklist and text position (not sure what this is called but you can center text or have it line up on the right side instead of the standard left). But no highlight or underscore. And I think that's really a shame because it's a feature I use very often.