Are there any professional poker players that play exclusively heads-up NLHE cash games nowadays?
If so:
How much do they win online yearly?
Where do they play online?
How big of a problem are bots in heads-up NLHE cash games?

Probably a handful, but not a lot of HU exclusive pros
It's hard to get volume since traffic for HU is low

It's pretty hard. The few hu cash guys I know and have heard of are all grinding multiple eurosites, maybe some app games, and waiting a long time for action. The stars hu zoom pools are decent for lowstakes play, but above 100 or maybe 200nl it can be difficult/slow to get action against weak players anywhere. Husngs are another format you could look at for some more action, but on most sites husngs are also a relatively dying grind and it can be rough above lowstakes with stuff like battling other regs for lobbies and low rec traffic.

Bots are definitely a concern and do exist in the space. Some sites do a better job staying on top of that stuff than others.

I think unless you become extremely strong at hu playing midstakes+ or have access to some great games that aren't so public, it's very difficult to win more than $40-$50k in a year playing hu of any kind now. There are surely a number of fulltime hunl or husng regs now who make less than that, and hu traffic is always declining year over year + sites regularly remove hu games, so longer-term prospects aren't particularly good either.

Sorry for the doom and gloom, but it's fair to say the state of hu poker right now is generally not good and the future is also generally not bright. You can still succeed playing hu, but would only advise if you really enjoy the gametype and are willing to work.

Its terrible. The worst its ever been. I had to start playing 6max again.

Being a HU specialist and waiting to get action possibly for hours on end is something I can't quite wrap my head around. I get that you'd probably open multiple HU tables on multiple sites and wait for an opponent to sit in against you, but what can you do while you're waiting? Do you need to be constantly looking at your monitor or can you do your chores and rely on poker table sounds etc. to notify you when an opponent sits in? For example, can you watch a movie on your computer in full screen mode while sitting in HU tables on the same computer? When an opponent finally sits against you at a table and you need to act, does that table window pop up and interrupt the movie you're currently watching?

Pretty sure some guys just open new 6max tables and wait for a fish to sit them and hope nobody notices before they bust them. As for table alerts, it's software specific. A pro HU player probably just idles the games on an extra monitor.