How to know whether SSD is permanently implanted on Mac or not?
The MacBook that has one USB-C port and the TouchBar MBP are the only Mac going back to 2013 models where the main storage is soldered to the logic board. All the rest are serviceable without solder or needing a new serialized logic board. 

That being said, Apple and certified repair agents will repair all Mac with storage issues for 5 years from the date of last sale of that model so practically, you’re never without repair options even if you don’t have the skill or tools or training to successfully close a Mac after repair. 

Just about anyone can go to ifixit.com and open up a Mac. The trick is getting the right parts, not damaging tiny screws and connectors and putting it all back together so the system is operational and reliable.