I had to leave my acoustic guitar at home. I had decreased the tension in the strings completely before leaving home...it has been 5 months...did I make a mistake?
Will my guitar still sound okay when I play it again?
Really, they're designed to be left under tension all the time.
The opposing forces of the neck [& truss rod, if appropriate] and the strings keep the neck straight.
Removing the strings will over time cause the neck to pull itself back - so when you re-string & tune it back to pitch, it's quite possible the neck will be out of alignment & you will get fret buzz.
Hopefully, leaving it in tune for a while will pull it back to where it should be. If it's bad, you could over-tighten by maybe a semitone [don't go silly with it, less is more ;) & see if in a couple of weeks it's starting to return to normal.
I wouldn't be inclined to take any steps other than that initially. Leave the truss rod alone, just use string tension, otherwise you may have to serially re-adjust as balance is reached, then overshot, then reached...
There was no need to slacken the strings. But you're unlikely to have done any harm.
I have a guitar in storage that belonged to a friend who died 9 years ago, almost to the day. (RIP CF.) I opened the case the other day. It's still in tune.
It'll be fine. When you get back to it, put some new strings on and tune it - you shouldn't have any issues. Any minor issues can be fixed by paying for a professional setup on it. Really, it probably won't even need that.
I had a guitar in storage for ten years, brought it to the USA and restrung it - it had no problems whatsoever.
with all my guitars which I don't play often then I detune half a step from standard tuning on all strings, I am sure it wouldn't hurt to leave it standard tuning, but I do it more for string life. with my daily driver guitar and the few guitars which I don't have cases for, then I leave them in the tuning I prefer for playing. the daily guitar i play is the variax standard which is always left in standard tuning. before that i would leave my cheaper Ibanez in standard tuning.
I think cleaning the neck and strings after play is far more important than worrying about strings. I use fast fret after playing, but also lemon oil to clean the fret board. though fretboard deep cleaning happens once a week on the guitar i use daily. On my other guitars I will clean the fretboard after playing a few times. I can go 6 or 7 months without playing some of my guitars.
[Edit] forgot to add that if I do need to do work on a guitar, then I will release the tension of the truss rod. but again it depends how long I expect it to be unstrung. E.G on one of my Bc rich guitars I changed the pickups, but due to work commitments I left the guitar unstrung for months, so I relaxed the tension of the truss rod so the neck couldn't bend backwards with the tension.
What you're doing in fact is a good practice.
If a guitar is going into storage for a long time, it's good to remove the strings, or at least severely loosen them to remove most of the tension on the neck, bridge and top. This is particularly a good idea for a steel-stringed acoustic with fairly heavy gauge strings on it (like a .012 or .013 high E).
If you're removing strings from a guitar that has had its truss rod tightened to correct excess neck relief, that should be loosened, too. Without any string tension acting opposite to the truss rod, the rod could create a back bow.
In a properly set up guitar, truss rods are always at least somewhat tightened so that they are firmly in place and do not rattle. For long-term storage, you can leave the truss rod lightly tightened, or you can remove all truss rod tension; in any case, don't forget about the existence of the rod when it comes time to string up again.
Some people don't believe in loosening string tension for storage: the thinking is that if a guitar which is played regularly can be kept in pitch, why would it be any different for a guitar in storage? It doesn't hurt, and could prolong the life of a cheaper instrument that isn't built well. I've seen cheap old guitars come out of years of storage with bulged out tops and bowed necks, exhibiting completely unplayable action. Of course, that will happen regardless of whether they are played or stored.