I got a good answer from the support (after some discussion why they should help me although my Matlab subscription has expired...):
This is XXXXX from MathWorks Technical Support, writing in reference to your Case #XXXXXXX regarding 'File Exchange Support needed'.
After working on this use-case with our Development team here are some information provided from their end that will be helpful to you in this regard.
1) To answer your question regarding people using older versions of MATLAB depends on how you define “old MATLAB releases. Based on your File Exchange entry, it looks like your app supports R2010a and above.
2) Unfortunately, the best support that we can provide today is suggest you to:
a) Lay out the files in a way that makes them work for users with R2010a-R2012a.
b) Package the app as an .mlappinstall file (William already does this).
c) In MATLAB, package all the code as a toolbox. The toolbox packaging UI should detect the .mlappinstall file and include it in the toolbox.
d) Upload the toolbox to File Exchange. File Exchange will offer the code as either a toolbox or a zip.
3) For PIVlab users:
a) Users with R2010a-R2012a should download the zip and use the unpackaged source code.
b) Users with R2012b-R2104a should download the zip and install the .mlappinstall file.
c) Users with R2014b and later should download and install the toolbox. Installing the toolbox will also install the app into the App Gallery.
Kindly note that there is a key difference between this process and what you are doing today (allowing File Exchange to create the toolbox file). If the toolbox is packaged in MATLAB, you can select the app in the packaging UI, which will auto-install the app when users install the toolbox.
Also, cc'ed in this email is XXXXX from Online Developer Workflows & File Exchange Team, whom can directly reach out to next time, when you have any questions regarding your File Exchange Submission.
As mentioned in my earlier email, as a frequent File Exchange user and contributor I see the importance of your request and me along with my team collaborated with the corresponding Developers to bring to you this information. I believe this helps you in keep contributing to our MATLAB community.
Once again, a gentle reminder about your license #XXXXXXX which is out of maintenance.