No idea why there isn't a warning in service letters.
It is in the operating manual, page 10-8. "Due to its high lead content AVGAS forms increased lead deposits in the combustion chamber and lead sediments in the oil system. Therefore use AVGAS only if you encounter problems with vapour lock or if the other fuel types are not available."

Having said that, I'm glad you came out unscathed! I don't think the Rotax warning as printed, is sufficient. Engines running 100LL need to be operated differently, and that usually is never covered.

To burn off the lead oxide, the engine needs to be run at higher than idle speeds and shut down from a higher speed. Lead oxide builds up more and faster at lower temperatures.

Running rich exacerbates the buildup.

Higher temp plugs are also wise.

Mixing 100LL and auto fuel makes the problem much worse - you should only use one or the other. Auto fuel adds reactants that promote (help) 100LL build deposits.

Personally, I wouldn't use 100LL in any water cooled engine (with or without Decalin) - they generally don't run hot enough to effectively burn off the lead oxide (change it to lead bromide). IMHO