> Unfortunately, as near as I can tell there is no sensible way to
> prevent people from storing arbitrary data in witnesses ...

To prevent "from storing arbitrary data in witnesses" is the extreme
case of the size limit discussed in this thread. Let's consider it along
with other (less radical) options in order not to lose perspective, perhaps.

> ...without incentivizing even worse behavior and/or breaking
> legitimate use cases.

I can't find evidence that would support the hypothesis. There had not
been "even worse behavior and/or breaking legitimate use cases" observed
before witnesses inception. The measure would probably restore
incentives structure from the past.

As a matter of fact, it is the current incentive structure that poses
the problem - incentivizes worse behavior ("this sort of data is toxic
to the network") and breaks legitimate use cases like a simple transfer
of BTC.

> If we ban "useless data" then it would be easy for would-be data
> storers to instead embed their data inside "useful" data such as dummy
> signatures or public keys.

There is significant difference when storing data as dummy signatures
(or OP_RETURN) which is much more expensive than (discounted) witness.
Witness would not have been chosen as the storage of arbitrary data if
it cost as much as alternatives, e.g. OP_RETURN.

Also, banning "useless data" seems to be not the only option suggested
by the author who asked about imposing "a size limit similar to OP_RETURN".

> But from a technical point of view, I don't see any principled way to
> stop this.

Let's discuss ways that bring improvement rather than inexistence of a
perfect technical solution that would have stopped "toxic data"/"crap on
the chain". There are at least a few:
- https://github.com/bitcoin/bitcoin/pull/28408
- https://github.com/bitcoin/bitcoin/issues/29146
- deprecate OP_IF opcode.

I feel like the elephant in the room has been brought up. Do you want to
maintain Bitcoin without spam or a can't-stop-crap alternative, everybody?
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