It is safe to use a shower that hasn't been used in years as long as the plumbing is all good and no leaks and the shower is clean.
Just make sure the shower has been cleaned and all the mildew or mold if any is gone and make sure the shower has no leaks and if it has leaks they should be repaired before you turn on the shower.
If the shower hasn't been used in years it's a good idea to get access to behind the shower and look at the plumbing and turn the water on if it's been off and look for any leaks.
A bath uses more water than a 20 minute shower especially if you use a low flow shower head.
With a low flow shower head your 20 minute shower will only use around 20 gallons of water or sometimes less and a bath uses around 30 to 40 gallons of water.
However if your shower head is a 2 gallon per minute shower head then it could use more water than a bath taking a 20 minute shower or could turn out to be the same.
A 10 minute shower with a low flow shower head is around 5 to 8 gallons of water and with a high flow shower head the 10 minute shower is between 17 gallons to 20 gallons of water.
An average shower uses between 17 gallons to 20 gallons of water depending on the flow of your shower head and the length of your shower.
I've read my water meter before a shower and after as hower and my shower uses 20 gallons in a 10 minute shower but my shower head is a 2 gallon per minute shower head.
If you have a 1 gallon per minute shower head or low flow shower head then you may only use 10 gallons of water in the 10 minute shower or 20 gallons in a 20 minute shower.
Showers mostly use less gallons of water than a bath if you take shower showers and use a low flow shower heads.
I love the pressure of my higher flow shower head even when I use more water.
We have plenty of water here so there's need to worry about conserving water.
An average bathtub or bath uses around 30 gallons of water.
Showers are also cleaner than baths as with a bath you sit in the filth and showers rinse all the germs away.
The average American shower uses 17.2 gallons (65.1 liters) of water and lasts for 8.2 minutes at average flow rate of 2.1 gallons per minute (gpm) (7.9 lpm).