I think solo kill gold is somewhat fine, if you reduce the kill gold with assist, getting help for a kill that you most likely get anyway feels bad.
If you just cut out assist gold, any more supportive classes will loose alot income mid/lategame and playing as a team will feel bad.
i think the bigger issue is killstreak shutdown and assist streaks.
Back in the daym killstrak shutdowns would grant more money to those that actually are involved but non global and less overall, helping champs to come back in a loosing lane if their opponent oversteps and dies despide beeing 3/0 or something. It also made helping a loosing lane more attractive as jungler.
Even more important, assists would also countribute to the shutdown value, so a 0/0/6 would also give shutdown, which is was important for turning around games as a loosing team.
Since 5.22, assists no longer give any shutdown and shotdown gold is golbaly distributed.
This means if i crush toplane and get shut down by their jungler, their bot profits from me doing well. They get gold because i got kills and then died. If i didn't get a couple of solo kills, they would not profit from me dieing top. Given how much more botlane scales of gold and how much more impact they have later on, this means that getting a solojill can be strategically the wrong call, and that feels horrible.
It also means that i will continue to somp the lane as my direct opponent doesn't get as much profit as befor and most likely stays behind, which makes the lane in it's own more snowbally.
The lack of assist shutdown means if you spread out the kills over your team and just gang up on opponents, they gain less shutdown than if a single player goes rampage, especially active junglers can get alot impact and if they mostly get assists, they don't have to care about giveing away shutdown gold at all.
I think what needs to be done regarding solo kills vs assisted kills is that assists should count as 0.5kills towards the shutdown bounty, just how they are rewarded 0.5 the kill bounty.
Then there are two options to handle the shutdown bounty: either bring it back to s imple increase of the kill bounty that is spreat localy amongst killer and assists rather than global, or keep it global but also reward kill/assist streaks with global gold for your team, so if i get 3 kills as a sololaner, the rest of my team allso gets a small gold advantage and the opponents can shut me down to balance it out.
It would also be interesting to get a 'revenge' mechanic that gives a small extra reward for anyone that was killed by me that is invloved in takeing me down, stacking up, so shutdowns can help bringing a loosing lane back on pair some more.
