After the SRAM 10-42 cassette came out on the market, several companies started selling larger replacement cogs for people to convert their drivetrains to a 1x setup. Some of the kits were just one, two, or three cogs, others included upgrades to the rear derailleur. Some people reported they thought the shifting was okay, others said jumping to a substantially larger cog was a bit rough. You can search around and read the reviews yourself. However, I guess most of those conversions were 42T or 44T options. 46T is pretty big.
E-thirteen had some options, Wolftooth, and if I remember correctly, OneUp had some of the better reviews. I believe OneUp also sells longer cages for use with the big cogs. Not all of them even expect you to have an 11-spd rear derailleur so I guess that would mean you can use an existing 9-10 spd derailleur with the larger cogs. Although, I would think you would need a long cage derailleur to take up the extra chain slack you will have.
For your question if a new 11-spd derailleur would work with a 46T cassette, my guess is probably, you might just need to adjust your b-screw tension until you get the upper jockey wheel far enough away from the large cog, or you might need one of those OneUp cages. You may not get the 12mm gap SRAM recommends, but it might work anyway.
