[LukaAl]

No. Imagine tomorrow a new RFC suggest to use the reserved bit for something meaningful. Any implementation that check that bit and drop the packet if the bit is 1 is broken immediately, even if it is not impacted.

True story, some years ago I had to design a protocol for remote configuration using text messages. We used a char to send the status in a compact format but in the first version we used only the first 3 bit and we specified that all the other bit should be zero (we have done so in order to have a predictable default in the future and also to increase the readability of the logs). After few months in the field we noticed that using an additional bit would be useful. Don't want to go into many details, not a problem if the bit is ignored but nice to have to simplify the operations. We updated the protocol, issued the new specs, the vendors develop the new version, we test the new version and surprise: the IOT between the new server and the old client was broken. Why? Because the designer the implementer of the client was to strict on the must be zero clause and decided that a packet with a one was a corrupt packet and should be dropped.

The result was a slow campaign of firmware upgrade and a 1 year delay of introduction of the new protocol. And it was a closed system where we had full control of clients and servers. Imagine what happens with open systems.

By the way, it is an application of this principle: https://en.wikipedia.org/wiki/Robustness_principle