Freestyle solos are a casualty of the game not attracting more casual players. It's a feature built for people who just pick up the guitar at a party and want to hit buttons and have fun. The RB purist was never going to take to it, and that's what most of the audience is now so freestyle solos have, as expected, been killed off.
I'd argue the sound quality of the freestyle solos had an effect too, but probably not enough to really cause a big swing in the number of people using the feature.
Speaking more broadly, it's almost funny how many things were talked about pre-release and/or implemented that, having supposed to have set RB4 apart from the other games, have instead been greatly reduced or quietly taken outside the shed and put down. I don't know what changed but we can see from those minimized RB4 features, to the indifference people have toward Seasons Rewards, to the mild enthusiasm for these Note Track Skins the Harmonix of the last three years has just been incredibly out of touch with the customer base.
i suppose it made sense to cater to the casual crowd when this game was first released. those people were the ones who would pick it up, play for a few months, then move on to other things. if a few shiny new gimmicks like freestyle solos inspired some extra purchases, then it was probably worth it. 3 years later, the game has returned to its niche status with mostly just the hardcore playing on any kind of regular basis, and little hope for pulling in new players when instruments are so expensive and/or hard to find.
it's just a shame that the initial focus was SO geared towards casual players over hardcore fans. basic things like practice mode being left out initially, and the long delayed wait for online play (followed by another wait to be able to play online with randoms) have made it a frustrating experience for the rest of us. even 3 years later, after all of the updates, the game still feels a bit half-baked. when we finally get a much needed or requested feature back, it's missing key ingredients, or just misses the mark entirely. we get practice mode back, but it STILL doesn't go slower than 70%, and even worse, you have to back all the way out to the main menu to get to it, when previous games allowed you to go straight into practice mode from the main quickplay menu. or take the new track skins.... people have been asking for customization options like note tracks and venue select for years, and they still get it wrong. putting goofy graphics beneath the track is NOT what we've been requesting all these years... it's the choice of note styles themselves that are wanted. and venue select is still nowhere in sight.
the release version of RB4 is what you get when your design and dev team thinks they're smarter than the audience who have been playing the game for years and have been telling you clearly and repeatedly what they want. you get a game full of features and upgrades nobody wanted or asked for, and missing key features the players expected or assumed would be included. and what RB4 has since become is what you get when the same design and dev team tries to play catch up after the fact with limited resources to make up for all the poor or misguided decisions that got them to that point.