Is it okay to abandon bot laine?

No, it's not. Abandoning your bot laner will only make things worse.

Midlaners don't want a support. They don't need a support. Your presence actually hurts them, because even if you help them secure a 2v1 kill, you split the XP that they need. Roam there and help them snowball, but leave as soon as you secure a kill.

Being the support is being the babysitter. It's not the jungler's job to keep your marksman alive; it's yours. Depending on the type of support you're playing, you're either bodyguarding that baby marksman, healing them and protecting them with spells, spoonfeeding them kills with CC, or zoning the enemy with high-damage spells. Marksman scale. If they're doing very poorly in the early game, they can still do pretty alright in the late game if you stick with them and try your best to keep them alive.

As an ADC main, I can guarantee that it's 100% fine to leave your adc in lane. The biggest problem is knowing WHEN to do it. If your adc isn't doing to hot, it's perfectly fine to go and help other lanes, but only if you'll actually be able to help. Like some of the other people have said, you may not want to go to a lane if your mates don't want you to. You can even ask the others if they need/want assistance be it through vision, some pressure, or a full gank. (please at least tell your adc if you're doing this though. Otherwise we're just left to kinda think in you're a dick)

Certain matchups are really bad to leave solo, such as Kai'Sa or Twitch, as they'll just be hammered over and over again. Champs like Sivir or Lucian can be fine, however, since they have a form of wave clear or safety on their own. Leaving your adc can also help get them back in since they'll be getting solo xp, as well. By roaming around, you might even draw the enemy support to other lanes, leaving the 2 adcs alone in bot which is far less likely to be volatile and put your struggling adc further behind.

I played a ranked game earlier today, actually, where i was on sivir but had an especially rough laning phase vs a cait/morg. Shortly into the game, my thresh left lane and helped around the map. He got our mid and jg some kills within a few minutes and actually forced the morgana to try and match him in terms of pressure so I only had to deal with cait in lane. It's much easier to farm up and catch up when you only have 1 champ trying to beat you down. Everything went well from there, (aside from thresh trying to flame me) I ended up getting my items in a timely manner to help out my team by the mid/late game and went on to win it.

I think the biggest risk, however, is the fact that most adc mains, especcially in low elo, are very prone to tilting if their support leaves them. I learned pretty early on that losing is a part of winning. You can't win every lane, every match-up, or every fight, but you have to know how to lose gracefully and minimize casualties. That's something that a lot of people still don't get and so they'll just run it down and blame everyone else.

In the end, it really comes down to the team you have. If they are willing to be helped and if your adc is equipped enough to not die and farm up for fights.

I hoped this helped! Sorry that it kinda turned into a wall of text.

You shouldn’t hang out mid if your midlaner doesn’t want you there.

If mid lane is doing well, going there may not be the best thing to do if your mid laner doesn't want you there. For one, you will eat his xp and possibly cs. For another, you will draw the enemy support and/or jungle to camp his mid lane. Also if mid laner doesn't want you and you go anyway, it may tilt him and cause him to rage. That's not selfish.

The problem with that is most of the times the suppprt feeds then gets mad and leaves their bot lane to fend for themselves. Meanwhile when you choose to leave your marksman in a 1 v 2 you are saying that you don't care if your marksman stays useless for the whole game, so you are gonna go steal exp from one of your solo laners putting them further behind their lane opponent. Chance are their bot lane is fed as fuck so now you have one person who can't even lane because its 2 v 1. Just think about the consequences of your selfishness before you leave your marksman alone and basically lose the game even harder. Most supports don't actually know how to support at low elo amd tend to do stupid things. And when their stupidity causes them to fail they get mad and ruin the game for the whole team.