A mate of mine uses an old pair of road shoes with an SPD cleat on his gravel bike; his shoes have 2 & 3-bolt pattern.
The SPD cleat sticks out a long way from the completely flat, slippery sole. They make a horrible crunching racket on stone/hard ground and look horrible to walk in. MTB shoes have the rubbery sole built up around the cleat, so this isn’t an issue. And when using SPD-SL cleats you have a bit more of a flat platform to walk around on.
Saying all that – I have just had a look and they do an adaptor, SM-SH41 which has little rubber bumpers either side of the cleat placement. Is that the one you are referring to?
That would actually make it a lot better than i have described above, as you won’t be walking directly on the cleat.
https://ride.shimano.com/blogs/technologies/how-to-use-sm-sh41-cleat-adapters