Is the sudo bless --mount command in macOS deprecated. If so, what is in use now?
No.  There is nothing (that I could find) from Apple indicating that bless has been deprecated.

The two articles you posted:


Don’t Use bless To Change Startup Disks Any More In OS X
Setboot? Formerly bless? Command line OS switching?


are a blogger's opinion about what to use instead of bless and a thread in an Apple Discussion forum in which a random person is asking the same question; the articles are from July 15th, 2015 and November 8th 2016 respectively.  Needless to say, they are woefully out of date.

This does serve as a opportunity, however to highlight an important factor:  Apple won't deprecate something (especially as critical as bless) without notifying the public about it (i.e. in the release notes, updated man pages, etc.)

Why?  Well, how would devs and sys admins be aware of the changes and begin planning their transitions?  

Deprecation notifications will come from Apple, not random people opining on the Internet.