Oh yes you can find it on the github site, but that means that you would have to install it yourself which 3DR has said will void your warranty. 3DR has a duty to release firmware updates that relate to flight safety. Especially if they are using them in their Enterprise operations.
Where did you see that the ESC's have been modified?
Of note are the comments in the diffs from:
1.5.0 fixed a bug where an incomplete compass cal would lock up subsequent attempts
fixed a bug where an EKF or GPS glitch failsafe recovery could override a battery failsafe
1.4.8 increase min pwm output when thrust a priority to prevent uncontrolled descents
1.4.6 improved land detector to help prevent tipovers
1.4.4 fixed a bug where multiple GPS glitches during a flight could lead to incorrect failsafe behavior
1.4.1 added a 2 stage land detector that will ensure that the landed maybe phase is triggered. This engages logic that limits max ESC output to prevent flipovers (false negatives on landing)
All of these relate to problems folks have seen with their Solos and relate to flight safety.
"override a battery failsafe"
"prevent uncontrolled descents"
"lead to incorrect failsafe behavior"
should be a clue as to the seriousness of the need for maintaining up to date firmware for the flight hardware.