You will see both. i prefer the 1990s because it's not possessive.https://englishlanguage101.wordpress.com/tag/1990s-or-1990s/

The second option

1990s ... NO apostrophe. You will see 1990's, 1000's of CD's for sale and even log's for sale. All are incorrect. 1990s. 1000s. CDs. Logs. These are the correct forms. Please check it out with The Apostrophe Protection Society. http://www.apostrophe.org.uk/page2.html

You will see both because the grammatical rule was recently change so that 1990s is correct. Both can still be found because the change was very recent but if you are writing about the time period, 1990s is right.

You see both. However, the second is preferable, because there is no logical reason to use an apostrophe here. We use apostrophes for possessive forms and to indicate that part of a word has been omitted. Neither is the case here. It is a straightforward plural - 1990s. There is no need for an apostrophe.