Gosh, guys.
My dad would mock all of you.
He had a Grade VI education and was anal about grammar - caught every little thing.
He would read the dictionary (as does my 11-year old ????) - who does that?
Anyways, 2 points. The book of Psalms does NOT have chapters, so saying Psalms chapter 6 is wrong. It would be the same as saying "Let's stand and sing Song chapter 6."
Likewise, we are usually referring to one Psalm at a time.
For several years, the WT articles have spelled Psalm, instead of abbreviating it, to try and help pronunciation.
The new NWT is done correctly.
If there is 1 Psalm on a page, it says Psalm at the top.
If there is more than 1 Psalm on a page, it says Psalms.