BootstrapVue is currently an implementation of Bootstrap 4.5 on Vue 2.6.
It's not using Vue 3.
It's not using Bootstrap 5.
Discussion around a Vue 3 + Bootstrap 4 version here.
Authors also mention work on a Bootstrap 5 + Vue 3 has not started yet but it's planned, on a separate branch.
Should you learn this version of BootstrapVue? The concepts won't change much, but the implementation might, (not so much around template syntax, but more on controller syntax - around object vs composition API usage).
On the other hand, one could argue if you know Vue 2 object API syntax and have reasonable knowledge of Bootstrap 4, there's hardly anything to learn about BootstrapVue.
Currently, if a company wants to use BootstrapVue in production, they'll use Vue 2.6.12 and Bootstrap v4.5.3, even if they plan on refactoring for Vue 3 + Bootstrap 5 at some later point, when they become production ready.
The vast majority of companies using Vue right now are using Vue 2 and, because no major UI library (BootstrapVue, Vuetify) has released a Vue 3 version, the only option for using Vue 3 is writing your own CSS/theme, which most don't deem worth investing in.
Getting back to your question, it roughly translates into: should I learn Vue 2 and Bootstrap 4? My personal answer is: "Definitely." They will be in usage for many years to come, even after a BootstrapVue version for Vue 3 + Bootstrap 5 will be out.
Also, there's a high probability having knowledge about how current version works will give you a significant advantage in learning and understanding future versions.
Minor note: you start your question by saying
"Since Bootstrap 5 no longer using jQuery..."
BootstrapVue means Bootstrap powered by Vue instead of jQuery. BootstrapVue has always been jQuery-less. That's the main reason why people use it: to get Bootstrap in Vue projects without jQuery dependency.
