To my knowledge, you can duct tape it to your leg if you want and it remains legal, you can even carry it in your hand, the only problem with carrying it in your hand is depending upon movements, finger placement etc it could become a brandishing charge.
I have open carried for several years and until last year when I got a revolver into the mix, I have NEVER used a strap type holster and have dealt with the police more than once.
I carry a full size 1911 in a Milt Sparks IWB holster that has no retention system beyond the tightness of the belt, I have carried XD 5" 45 in the factory, a crossbreed, and a SERPA, same with xd 9 sc, and have never had a gun try and leap for freedom
I would highly RECOMMEND a positive retention holster, I use them a lot. Since your firearm is on you for your personal protection, it means you are prepared for being criminally attacked. Since that is the case, then research of how criminals attack has wisdom. The Weaver stance, the draw and fire drills etc are all good, but if you consider your body to have 4 sides, you are only 25% likely to be face on with a criminal in an attack. If you get jumped from behind, attacked from the side or even have to run and fall, you do not want your firearm leaving your person and becoming available to the criminal.
This is why police carry in high level retention holsters, they are very likely to do any of those things on a given day, much more than we are, but that does not change the rules when it happens.
Required by law? Nope
Recommended by many? Yes
Decided by? You, just as it should be sir, just as it should be.
Good luck with your choices and there are a lot of good instructors working at ranges that will give you a lot of feed back, some will even let you try some of theirs at the range to help you make that choice. Most of us have a drawer or two full of crap holsters we bought on the down low cheap side, you never know, you might get a freebie from someone who hates it after purchase, I know I have given a few away.