Hello.
How do you think, will XF 2.1 will compatible with 2.0 themes and plugins? Or maybe developers сan answer this question.
Or there will be difference like between 1.5 and 2.0 (no compatibility).
Note that it's really dependent on the specific add-ons/styles and how fundamentally they change things or the level of interaction they have with changed areas. Some add-ons will need 0 changes; some styles will likely merge cleanly with any template changes we've made.
But it is absolutely not like 1.x vs 2.x -- it's more equivalent to 1.4 vs 1.5.