Yes, as an optional behaviour.
I've added the idea to the
wish list in case we decide to keep "steel is always steel" behaviour in SLX - it's definitely worth considering.
Three points of discussion:
1) This would essentially revert steel back to current NL behaviour whenever the flag is used. So, we'd end up with the best (and worst) of both worlds. An obvious drawback to this is never knowing what type of steel behaviour to expect, and that's even if the feature is never misused to intentionally troll the player with partially-destructible steel. The idea kind of negates the proposed "steel is always steel" behaviour, making it suddenly "steel is whatever the designer wants it to be"; not necessarily a bad thing, but whether or not it's a good thing depends very much on the player!
2) With that said, it's kind of
already possible to do this (i.e. with a "steel is always steel" system) by simply placing a terrain piece over a steel block, setting the terrain piece to "Erase", duplicating it in-place, then setting the duplicated piece to "Not Erase". To keep steel behaviour intact in such a scenario, we'd need to also check for erasing pieces and see if
they are completely obscured by terrain. Otherwise, we could end up with hidden, partially-erased steel blocks!
Note to self: It's currently possible to do this in the SLX working copy, so maybe the next release of SLX should keep NL steel behaviour until we can get a solution together for this particular problem...3) Maybe it's overcomplicating things? It's not difficult to create a custom steel piece which can be used as an eraser and then grouped with the visible steel piece to create a resulting single piece of the desired shape. Moreover, there are already dozens of steel pieces which have pretty much every possible shape covered between them, without even needing to create any more.
Erasing and grouping has revolutionised what's possible in the Editor; I almost feel like it'd be more worthwhile (and, frankly, much less effort) to add some generic-shaped steel pieces to the default "Special" style (triangles, squares, circles, etc) so that they can be used for the type of case you're describing. In the event that we go ahead with "steel is always steel", would that satisfy?