So I have read plenty of stories on this sub regarding managers waiting until the last possible minute to try and train a replacement because nobody thought to think ahead. I had it a little bit different in that I was the only guy who knew how to do not only my main duties, but also about half a dozen additional things and I tried way too hard to get others trained to do it all so it wasn't all on me. Not including my normal duties, the extras that I did saved the company about $4-5k per month because I was able to do them in-house rather than contract it out (it also saved weeks of time.)
Prime example: imagine swag for businesses. They have an internal (or external) website to showcase what everyone can buy that had the company's logo splashed on everything. We would need images created to showcase these items just like every other retail website out there. So I would grab the already made blank stock photo of the item from the manufacturer (think a Nike Polo), plop that into photoshop. Grab the customer's logo, plop that into photoshop, and resize it on top of the item to the correct dimensions. Export the various sizes for the website and send to the web team. Not only does it look cleaner than a final item being created and then photographed in a studio (2 week turn time), but I could do it in 5 minutes and never leave my chair.
I tried for 6 months to carve out training time to show others in my department how to do some of these things so I wasn't the only one relied on for it and every time my boss cancelled it or postponed it citing that it isn't priority. I finally gave up scheduling the training time and nobody mentioned it again.
Fast forward another 6 months and I am tired of my boss micromanaging everything (plus Covid hit and it brought out the real monster in her) and put my two weeks in for a better job. As I knew would happen, I was walked out by HR within about an hour of turning in my two weeks notice.
Two days later I get a call from the IT guy asking me how to do it all (because I was friends with him and they thought I would tell him). Told him that although we were friends I wasn't going to help as I don't work there anymore and am not getting paid. He didn't bat an eye because he knew what kind of crap I had been put through and said he thought I would say that.
Last I heard they had to outsource everything again and the previous company they were doing it through raised their rates and required upfront payment instead of Net billing. Also apparently my old boss was originally looking at retirement within the next year and had to cancel that because of all the work they now had to take over since I left.