My plugins have been focused on Intellij Community, but I do not restrict their use to Intellij Idea Ultimate (IU). The problem is that I do not have IU licenses, to guarantee an experience also in IU or impact the functionality of the bundled plugins.
What would be the way to test plugins in IU (10 minutes) via sandbox, when there is no license for all versions and the test licenses for the releases have already expired?
You can apply for open source licenses if you have a fairly actively maintained project (not necessarily the plugin).
I believe you can just target IU in your plug-in and then use the runIde gradle task to launch an ultimate version for testing.
EAP versions are also available for quite long periods of time during year
Yeah, I was confronted to this problem a few years ago. My understanding is:
You can apply for open source licenses if you have a fairly actively maintained project (not necessarily the plugin).
Absolutely. This is probably the best solution if your plugin is free (gratis) and open source.
Also, keep in mind that you can use the same license for your real IDE and for your sandboxed IDEs.
I believe you can just target
IUin your plug-in and then use therunIdegradle task to launch an ultimate version for testing.
Unfortunately, this won’t work. A paid IDE launched via the runIde command works like a regular paid IDE. That means your sandboxed IDE will use a trial license. Later, you will have to acquire a license, or your sandboxed IDE will shut down immediately (which is a bit useless 
). Actually, this make sens. Otherwise, everybody would use paid IDEs for free.
So, if your plugin is free & open source, you definitively should apply for an open source license. Remember, this works for your real IDE, and for your sandboxed IDEs.
Otherwise, if this is a paid plugin, I think a paid license worth the price.
This is what I did: I was developing free & open source plugins for years, and JetBrains offered me an open source license for the all their paid IDEs (aka the All Products Pack). The license renewal (each year) is fast and easy. Later, I started to sell some plugins, and I acquired a paid license for the All Products Pack. This way, I can test with any IDE.
EAP versions are also available for quite long periods of time during year
Absolutely. EAP IDEs are available ~11 months over the year. In practice, the EAP program is closed a few weeks during the summer (if my memory is correct). When the EAP program is closed (and when your last EAP trial ends), you cannot use them, even via the runIde.
I’d say this won’t be a problem for most plugin developers.
The problem for me: there is no EAP (free) build for previous IDEs 
(and this is normal). So, with EAPs, you can only test with the very latest IDE builds. This is why I’m happy with the All Products Pack. I can test with any IDE, and any version.