The question is has the rule changed?

Yes. For example, the Webster's New Collegiate Dictionary, 1981 edition, gives this rule in the section “Handbook of Style”:

###Apostrophe ’

• often forms plurals of letters, figures, and words referred to as words
  ⟨You should dot your i's and cross your t's.⟩
  ⟨His l's and his 7's looked alike.⟩
  ⟨She has trouble pronouncing her the's.⟩

No examples or guidance are given for omitting the apostrophe. This is different from the common advice today to use omit the apostrophe except when unavoidable.

The question is has the rule changed?

Yes. For example, the Webster’s New Collegiate Dictionary, 1981 edition, gives this rule in the section “Handbook of Style”:

###Apostrophe ’

• often forms plurals of letters, figures, and words referred to as words
  ⟨You should dot your i’s and cross your t’s.⟩
  ⟨His l’s and his 7’s looked alike.⟩
  ⟨She has trouble pronouncing her the’s.⟩

No examples or guidance are given for omitting the apostrophe. This is different from the common advice today to use omit the apostrophe except when unavoidable.

The question is has the rule changed?

Yes. For example, the Webster's New Collegiate Dictionary, 1981 edition, gives this rule in the section “Handbook of Style”:

###Apostrophe ’

• often forms plurals of letters, figures, and words referred to as words
  ⟨You should dot your i's and cross your t's.⟩
  ⟨His l's and his 7's looked alike.⟩
  ⟨She has trouble pronouncing her the's.⟩

No examples or guidance are given for omitting the apostrophe. This is different from the common advice today to use omit the apostrophe except when unavoidable.

The question is has the rule changed?

Yes. For example, the Webster's New Collegiate Dictionary, 1981 edition, gives this rule in the section “Handbook of Style”:

###Apostrophe ’

• often forms plurals of letters, figures, and words referred to as words
  ⟨You should dot your i's and cross your t's.⟩
  ⟨His l's and his 7's looked alike.⟩
  ⟨She has trouble pronouncing her the's.⟩

No examples or guidance are given for omitting the apostrophe. This is different from the common advice today to use omit the apostrophe except when unavoidable.