By Old School Rules, I mean rules that I though would make 5e more Old School. And *spoiler alert* they didn't work. But I'll let you know what they were, why decided on them, and why I think they didn't work.
We used a mod that long rests don't auto heal, but I modified it some. Base HP was Con + HD + mods, of which your Con represented your actual physical ability to take damage. So HD were the abstract portion of the HP. Once you started taking damage that went below your Con your were taking physical damage, in essence--bloodied. That could only be healed magically or on a table I had devised. You could still regain HD and heal via HD, which represented your ability to bounce back from combat stress and fatigue and be back on your A game. But that nasty belly cut you got in the last battle--yeah, you're still nursing that. Not a perfect system, but it was a compromise between player ability to withstand damage and the notion that combat dealt more than abstract hits. This was important mainly for the second rule...
No death saves. If you got to zero you were going to bleed out in 1d4 rounds + your Con mod. This was a big blow. Because, honestly the death rate has been very low in my campaign so far because of those blasted death saves! I have only had one PC actually die in two out of three death saves. Which, by my calculations is actually much higher than what is supposed to be statistically happening. And few Players even roll three saves, since some other PC usually uses an individual or party heal that spares them. My experience is that it's hard to die in 5e--am I doing something wrong? Anyway the HP rules above were a compromise in making PCs a bit tougher in the HP area, while realizing that if they drop to zero they ar more than likely to die.
Shields splinter and armor breaks. As per Ubiquitous Rat we used these rules almost verbatim. I really liked them in print, not so much in play--but I'll come to that in a moment. We also used the spell acquisition rule that wizards had to find spells to acquire them and not acquire them automatically. We also used critical hit and fumble tables, but this is somewhat of a norm at our table anyway.
Now, having said all that I did allow 4d6 drop the lowest arrange as your like, and I gave them inspirations points to start with, and allowed them to have feats. I know, I know, not at all old school. Well, I truly expected a high death count, and wanted to blunt the edge to the moral blow to their power quotient the new "old school" rules would impose. Turns out this was wrong on both counts.
So, how did it go? Well, the new healing rules were a pain to implement. Required way too much in game tracking, and my players, being somewhat younger generally and relatively new to RPGs were not fond of or adept at the extra record keeping. Also, healing was still sufficient to avoid too much trouble for them, and the cleric and other healing classes (grrr) just made sure to compensate. So we literally had no death saves to make--it seemed that the game was a tad grittier, but the rules would need modified to work fluidly. Maybe more work with them would have helped, but frankly, I felt like I was tacking on a clunky addition to a game that simply wasn't designed to work that way.
The shield splinter and armor breaks rules also became clunky--same problem as above. And we quickly dropped them. I dropped the healing issues next, as I found I was having to police HP more than the players were. As we dropped these about the third session we also dropped the inspirations points. I didn't take their feats, but we made it clear we wouldn't be playing with them for future characters.
So we went back to a pretty much regular 5e game, and pressed ahead. We are now over a dozen sessions in and they are fourth level--another thing I hate--how quickly low level characters zoom to mid levels. And what is my conclusion?
I'm sure a group could play 5e what might be called classically old school. But, it aint easy, and the game isn't really designed to do that. I am just now wrapping my brain around bounded accuracy, and I can't say I like it all that much. So, here I am again. Not pleased with 5e as a game itself, and it is just that: a different game from every game that has come before. It is not like AD&D, nor like 3.5 or like 4e. It is a rebranded package designed to appeal to the lowest common denominator in gaming. I personally think that is why so many of the old school crowd has picked it up--it is "sort of" old school, at least when compared to the last couple of editions. And I think those who did like 3.5, Pathfinder, and 4e, find enough of 5e likable that they are okay playing it as well. Maybe that is what the company aimed for--hit the broadest swath of gaming culture they could. Problem is, they have alienated the fringe. And I am on that fringe. I tried it, and I am not liking it. I will say, I have given it longer than any other edition I have played thus far except of course AD&D. But it is about time to change that.