I'll merge this, but I'll wait until #4 is done to release this PR along with #4 in DashBase v0.3.0

in DashBase v0.3.0

There are no breaking changes in any if those PRs, so this should be released as 0.2.1

If you were to make a breaking release, I'd vote for making that 1.0.0.

From the perspective of DashBase.jl, you're absolutely right. We're just adding things here.

But, what would happen if we release DashBase v0.2.1 off the current master, then users installing or updating Dash.jl would get DashBase v0.2.1 along with Dash v1.3.0 meaning that Base.getindex(component::Component, id::AbstractString) would be defined in two places. Doesn't that lead to errors?

If you were to make a breaking release, I'd vote for making that 1.0.0.

Yep, we can make the next DashBase release v1.0.0

We'd need to test that precompilation does not break, but I'd think that those methods would just get overridden, which is not ideal, since you'd get the buggy version then.
On the other hand that is only an issue until the new Dash version is released. But then, if Dash is the only consumer of DashBase, making it 1.0 is also fine.