terminal – Is the sudo bless –mount command out of date in macOS? If so, what is currently in use?

No. There is nothing from Apple to suggest this bless became obsolete.

The two articles you posted:

are a blogger’s opinion of what to use instead bless and a thread in an Apple discussion forum where a random person asks the same question; The articles are dated July 15, 2015 and November 8, 2016. Needless to say, they are absolutely out of date.

However, this is an opportunity to highlight one important factor: Apple won't disapprove of something (especially not as critical as bless) without informing the public about it (i.e. in the release notes, updated man pages, etc.)

Why? How would developers and system administrators be aware of the changes and start planning their transitions?

Expiry notifications come from Apple, not from random people who are on the Internet,