I'm not an IP lawyer, but a safe guess would be yes, it will remain free and open source, and an even safer guess would be that Lubos most likely wouldn't make it closed source anyway, which is also what the Patreon is about.
License wise, Armory is using zlib, which states:
This software is provided 'as-is', without any express or implied
warranty. In no event will the authors be held liable for any damages
arising from the use of this software.
Permission is granted to anyone to use this software for any purpose,
including commercial applications, and to alter it and redistribute it
freely, subject to the following restrictions:
1. The origin of this software must not be misrepresented; you must not
claim that you wrote the original software. If you use this software
in a product, an acknowledgment in the product documentation would be
appreciated but is not required.
2. Altered source versions must be plainly marked as such, and must not be
misrepresented as being the original software.
3. This notice may not be removed or altered from any source distribution.
Essentially, you're free to make a completely free fork of the engine right now if you want to on Github, and the only requirement is that you can't say you wrote the original software, and that you don't even have to acknowledge Lubos, even though I'm sure it would be appreciated.
So if you're planning on making a game with the engine, but are afraid it'll go closed source or become commercial software? - Then I'm sure there's no need to worry, at least with Armory it should not be those worries, which hinders you in creating your own fun game project, but rather your creativity, motivation and imagination