11-42t cassette with a 9 speed derailleur, will it work?

A 9 speed Deore groupset is presumably a 590, 530 or 510 series. Assume you have a 590 series which was made up to 2009 (the Shimano Wikipedia page has a nice table of group series and years), you can pull the Shimano 2009-2010 specifications from this page

Search in the PDF doc for 'RD-M59' and you'll find the RD-M592 model has a max sprocket size of 32 teeth and the RD-M591 model max sprocket of 28.

So technically no, your derailleur will not work with a 42 tooth cassette, however, many people seem to have had success with a derailleur hanger extension that drops the derailleur down so it clears the largest sprockets. The derailleur has enough total capacity (45 teeth difference for both models) to take up the chain slack if you will be running this with a single front ring.

Yes, but... You will need to use this handy gadget from Wolftooth called a RoadLink. It moves the derailluer a bit further away from the bike to allow clearance for a 42t cog. They sell a similar device called a Goatlink, but that won't work with your derailleur. The RoadLink works with any derailleur using a standard mount and is not Road specific in any way.

I successfully used this setup to shift an 11-40 9 spd cluster. 42t is will probably work, but anything larger is unlikely. You might also need to buy a longer B screw for the derailluer.

I'm currently running a Deore M592 on my 1x9 gravel 26er gravel conversion. 44t NW chainring & 11-40t cassette, no roadlink needed & I'm sure I could get a 42t cog in there with a roadlink, but that would definitely be maxing out the derailleur capacity with out reducing chainring size.

I am using m951 with 11-40t cassette, works flawlessly. I am really happy with it. Might even work with 11-42t, but I didn't try.