look at MUDs. some of those are fully programmable. i am most familiar with LPmuds, which are written in LPC, which is a complete programming language. (Pike a general purpose programming language is derived from LPC. and there are also a few small MUD engines written in Pike)
though for a standalone game a full MUD may a bit much since they are designed for large words with hundreds or thousands of players. but take a look at the available game engines and see which ones come with decent documentation for you to get started easily.
Can you recommend any mud “engines” that can be used on a modern linux system which are easy enough to get into (both for users and devs) and battle tested for security?
i am afraid that this is rather difficult to answer. the first question is where you really want to go with this?
some of the issues to consider are:
do you want to potentially commercialize the game? that rules out some engines (most notably some LPmuds) because their license only allows noncommercial use.
then you have to consider the size of your future game. some MUDs are huge and their code has a long and checkered history. while they are more likely to be battle tested, i don't know if a huge engine would be a good start for a new game because it would prescribe a lot of game mechanics already, and may not be so easy to get started with.
personally i would rather opt for a small engine that doesn't have more than basic game mechanics so that you are free to develop your own. but those are most certainly not battle tested, and there are probably quite a few that were started but are now abandoned.
finding a small, active and tested engine may not even be possible.
i would recommend to actually play and build with a couple of MUDs that are based on potentially interesting engines to familiarize yourself with how it works and see if that is something that you'd be comfortable to work with, and then take it from there.
though for a standalone game a full MUD may a bit much since they are designed for large words with hundreds or thousands of players. but take a look at the available game engines and see which ones come with decent documentation for you to get started easily.