It's true that you can get hearing loss from noises over 85 decibels, but the length of time you're exposed to those sounds is what matters. A gunshot is only a split second. So a few shots from a suppressed rifle that are under 140 decibels are considered hearing safe. If you're going to shoot more than a few shots, then you still need hearing protection. And if you shoot a suppressed AR-15, the sound at your ear will be over 140 even if the sound at the muzzle is under 140, because of the sound coming out of the ejection port, so you should always use hearing protection on an AR-15 or other semi-automatic rifle. There's a helpful chart I found here: https://www.noisehelp.com/noise-dose.html
Maximum Recommended Noise Dose
Exposure Levels
Noise Level (dBA) Maximum Exposure Time per 24 Hours
85 8 hours
88 4 hours
91 2 hours
94 1 hour
97 30 minutes
100 15 minutes
103 7.5 minutes
106 3.7 minutes
109 112 seconds
112 56 seconds
115 28 seconds
118 14 seconds
121 7 seconds
124 3 seconds
127 1 second
130–140 less than 1 second
140 NO EXPOSURE
Maximum Recommended Noise Dose
Exposure Levels
Noise Level (dBA) Maximum Exposure Time per 24 Hours
85 8 hours
88 4 hours
91 2 hours
94 1 hour
97 30 minutes
100 15 minutes
103 7.5 minutes
106 3.7 minutes
109 112 seconds
112 56 seconds
115 28 seconds
118 14 seconds
121 7 seconds
124 3 seconds
127 1 second
130–140 less than 1 second
140 NO EXPOSURE