I've 3d printed many brackets, adapters, covers, connectors, GPU supports, etc for many servers and desktops. There's no issue with fire.Maybe 3D Printing but I would be carefull if something start to Burn it can mabe ignite and you get problem with your insurance.
I actually take great pride in that when I run across an adapter or bracket I need, instead of shelving the project for a week or two while I browse eBay looking for this $5 or $10 tiny metal bracket, I go searching for existing brackets or at least something close to what I need, print it same day (usually a couple of hours) and I continue the build. Usually on www.thangs.com (which is an 3d model listing aggregate site). If that bracket or adapter or cover doesn't exist, I take one close to it and "kit bash" a new model out. Kit bashing is kind of fun actually once you get into it (where you take existing models into a Mesh editor, and pull, poke, push you away to a new design, then export and print.
However, depending how "hot" you run your servers, PLA (the most common 3d printer filament) may get ever so slightly "soft" if you have massive heat escaping (think 3 or 4 GPUs' heat). This is not an issue with any normal airflow you have in chassis and cases.
For example, I have 3 brackets printed in PLA installed in a SC846. It has 24x HDDs (that are not spinning down), dual 40G nics, SAS3 8 port HBA controller (very hot), U2 drives, etc. This is a very warm chassis and heats up the server closet way too fast (no exhaust). However, the PLA brackets remain very stiff and never get anywhere near warm enough.
My point is, PLA (the single most common 3d printing filament) is more than fine for PCIe brackets. And it had some of the lowest glassification temps of 60C (140F) as well (meaning it starts to get slightly soft at temps as low as 60 to 70C). Meaning, if PLA works, than any filament will work (cause most others like PETg and ABS have a much higher glassification temp).
There is no fire danger with 3d printed parts in servers.
Just note that if you do print 3.5" to 2.5" adapters, and you use them on some very hot SSDs (Optane, NVMe drives, etc), then you can get some slight deformation from the heat with PLA adapters. While I've never had issues, I've noticed a few "very tight" screws that shows ever so slight molding of the drive into the plastic, small indentations, which indicates the plastic got a little too warm.
I print anything that is going to touch an NVMe drive in PETg and ABS now. Any other drives, PLA is fine.
I'd be more than happy to 3d CAD design a bracket for these, if someone wants to send me a card.