Is dGPU disabler permanent on 2011 MBP? I've found this tool (http://dosdude1.com/gpudisable/) to disable dedicated GPU on my MBP late 2011 (I just bought it from eBay). The Mac has no problems right now but it's just preventive. I will only using it to browse the web and do some VM stuff, and I don't need extra GPU power.

Is it permanent (except when upgrading macOS) or semi permanent like some other solutions I've seen on the Internet (will the Mac reuse the dGPU after some reboot)?
That solution provided by dosdude1 is a combined package of software and hardware. The software part is of course reversible, and the hardware part of course not. Not easily at least.

With that hardware solution you will also have to take an extra step, as it will disable brightness control. That makes the hardware solution less desirable for a working MacBook.
If the MacBook will eventually fail, then that solution by dosdude1 is the second best option overall, right after replacing the chip itself with a working one from a line of known-good production.

As long as the machine does work, I'd say leave it at that. Do not disable the dGPU preemptively.

Preventive mesasures to prolong its life would be to increase the fan speed and other measures to increase cooling. That would include not using Thunderbolt and especially not an external monitor. You may also use gfxCardStatus to avoid switching to the dGPU as far as possible. 

If you insist on preemptively just disabling the dGPU you may also want to look at this option: GPU problem - Boot Hangs on Grey Screen