So if you simply use something without modifying it, you have nothing to do.
It's pretty well understood that non-GPL software cannot link to GPL-licensed libraries, which is why the LGPL exists. This is true even though most software doesn't need to vendor or otherwise modify upstream libraries.
Or does AGPL have a special carve out for this, different from the GPL?
Vitality comes from the way you combine this AGPL code with other code and how you can’t change the license of modified Plausible CE, or the thing you build using Plausible CE.
If you modify Plausible, you distribute the modified version. If you build something using Plausible, you distribute the thing, and with AGPL license. That’s it.
This doesn't make your site as a derivative of Plausible, but you're installing Plausible as a different tool, independent from your site.
Like installing a GPL binary doesn't make all BSD licensed binaries switch to GPL license on your system, running an AGPL licensed service on your server which communicates with your site doesn't make your site AGPL automatically, because you're not deriving Plausible, but you're installing and using it.
Now, if you want to build a service/website by directly modifying the Plausible CE source code, you have to open that derivative codebase with AGPL, only if you're redistributing the modified code (i.e. making the modified website available to other people). If you modified the code, but you're not distributing it (i.e. Only you're using it yourself), you can keep it to yourself.
But in the section that is the difference from GPL (section 13, "Remote Network Interaction"), it clearly states: "if you modify the Program, your modified version must ..." [emphasis added]. So the extra stuff that differentiates AGPL from GPL applies only for modified versions of the software.
For the rest, usual GPL stuff applies: if you distribute the binary, you must make the sources available, etc.
No, you don't. Folks pick AGPL because of the FUD/uncertainty around it while still appearing "FOSS". In reality these companies don't have the legal teeth to back the bark, otherwise they'd go BSL and make their intention known.
Before folks reply with whataboutism, no one has ever successfully proven the AGPL infectiousness to other web applications in court. Positing about it on hear is pointless.
Or alternatively, they still want to be FOSS, which AGPL is, but BSL isn't.
This gets exploited all the time when companies use FOSS licenses to get traction, then pull the rug and switch to a more restrictive license once customers are locked in.
On the flip side, if a company tries to be honest from the beginning by using BSL (or similar) they get endless grief from religious zealots.
AGPL is the magic unicorn license that is just restrictive enough to protect you from your competitors, while still passing the FOSS religious purity test.
We trust Free Software not because of the license, but because of the ecosystem and number of eyeballs. Go tried to pull something similar by forcing telemetry on everyone, and they were almost burning everything down. Almost.
People/companies get vilified when they change licenses because of the way they do it. They don’t openly say that they need the money, did some mistake and need to change things to keep company afloat, but they also change their tone after license change and vilify the past contributors and become aggressive themselves.
There are many friendly open core or closed source companies out there, and I trust them with my code, data and secrets, and pay them without any hesitation.
Software ideologies are something, so the real world. While I’d love to see everything as Free Software, we’re not there yet, we might never be, and that’s OK. I’m writing Free Software and try to use Free Software as much as I can to show my colors and do my part in the thing I believe. But I’m never a mindless zealot spewing things.
What? I can't think of any examples where they don't quote money and the lost revenue from hyperscalers just taking their software and reselling it without contributing. InfluxData's CTO and co-founder even wrote a blog post on open source monetisation and how hard it is. Elastic, Mongo even explicitly pointed the finger at Amazon. Many others smaller ones did it preventively, like Sentry and Sourcegraph.
I've also never heard any vilifying of past contributors, outside of bickering about the meaning of open source (I think it was Mongo that claimed they're still open source with the SSPL license?).