My advice for riders in 2021 is pretty much the same as that I gave to riders before 2017. I am happy to report that many riders did take my advice and I saw fewer failures in 2017 compared to 2013 despite the 50% more riders.
My main advice is this:
Avoid using weight weenie low spoke-count wheelsets. (in fact any super-lightweight components should be avoided, many weight weenie super light components compromise durability for lightweight and thus are more prone to breaking)
Every rider should carry with them some spare spokes, however this is doubly important if you are running wheels with non-standard spokes. Had to replace one rear wheel this year due to a broken non-standard spoke. Also if your wheels have weird non-standard nipples then you should take the tool with you.
If you are running a frame with a replaceable derailleur hanger then you should take a spare with you, thankfully many riders this year did, I replaced 2 bent hangers where the rider had a spare on them. Hangers are normally specific to the frame so very unlikely to be found at a local bike shop. Hangers are designed to be weak so they bend/break off before the frame is damaged.
As per my advice after 2013, all riders should check the condition of their gear cables inside the STI shiftier for any fraying. Only had two cables broken inside shifters this year (down from 4-5 in 2013).
All riders should carry a spare folding tyre however this is double important if you are running some weird obscure size which is not readily available at local shops (yes, Stuart with the 451 wheel!)
At Louth one person did present with a flat Di2 battery however he was able to lend of a charger off another rider and charge it. It was an internal battery so we had to put the bike next to the socket which was a tad awkward. As per my advice from 2013, anyone running electric gears should take the charger with them, even if the manufacturer claims the battery will last long enough! Even better is a means of powering said charger without needing a mains socket (such as a power-bank).
Thankfully no-one presented with a broken internally routed cable, however if anyone had, I would not have spent hours replacing it, I would have just run a full length of housing from the shiftier to the derailleur and zip-tie it to the frame (I had a workshop roll of housing). They can then internally route it themselves after the event.