Another opinion...
From the Gregg Reference Manual:
When words taken from other parts of speech are used as nouns, they are usually pluralized by the addition of s or es.
Ifs, ands, or buts
Dos and don’ts
Yeses and nos
Ins and outs
Ups and downs
Yeas and nays
Pros and cons
Whereabouts
Whys and wherefores
Haves and havenots
If the pluralized form is unfamiliar or likely to be misread, use and apostrophe plus s to form the plural.
Which’s and wherefores
Or’s and nor’s
The Elements of Grammar, however, notes that nouns ending in vowel sounds could have an apostrophe added, as can plural reference to letters
A’s and b’s
Dot your I’s and cross your t’s
Do’s and don’ts
Oh’s and ah’s
Thank you, to me, ends in a vowel sound, so I vote for adding the apostrophe.
Let us, however, eschew the addition of an apostrophe with numbers and dates.
1990s
1800s
The 60s
Lucky 7s or sevens
Thank you’s to all.
PL