Oh man, I forgot about words like “flaming” and “flame wars”. These words sound so stupid in 2023. Let’s all agree to pretend we never said them.
Anyway, to throw in some more useless anecdotal evidence, yes - the Prequels were very controversial in the early 2000s to maybe around 2013 or so, at least on Star Wars Internet forums. Just go to any old school Star Wars Internet forum (including this one) and read posts from around 2001-2010 (if they go back that far) and you’ll find that almost every other thread is some heated argument about the Prequels. As Kellythatsit mentioned, there was even an entire stupid terminology used back then to describe both sides of the argument (“bashers” and “gushers”, oh god it’s embarrassing to even type those words now) because the phenomenon of arguing about the Prequels was so common.
Anyone who tells you otherwise, or tells you that the Internet loved the Prequels until Red Letter Media came along or anything similar is blatantly wrong.
Off the Internet, it’s much more difficult to gauge popular sentiment. But I recall that in the early 2000s the prevailing sentiment of popular culture towards the Prequels (or at least Phantom Menace) seemed to me at least to be mostly negative and prone to mockery.
For an example of early Prequel criticism (so early it was actually written before the Phantom Menace was released to the general public), see this 1999 review by Eli Roth (complete with dated 90s lingo), and also look at the comments section: http://web.archive.org/web/20000305191203/http://www.leisuresuit.net/Webzine/articles/starwars_rev.shtml
You’ll notice that even in 1999 - before the movie was even released to the general public - and a decade before RedLetterMedia, all the usual anti-Prequel talking points of today are fully present (over use of CGI, juvenile tone, Jar Jar, under-developed plot, breaking continuity with the OT, dull acting, etc.)
Regardless, I’ve met people in their late 20s that genuinely were surprised to learn that there was ever widespread dislike towards the Prequels on the Internet. There’s obviously a generational aspect to this, and I can understand why younger people might be skeptical about this, since it’s so different than the prevailing attitude today.