Lew Zealand
Helper
Both Steam and GOG have older v1.5.6 versions of Stardew available for some platforms.
Steam on Windows:
Right click Stardew Valley and select Properties
Click Betas
Click the None popup next to Beta Participation
Choose legacy_1.5.6
gog.com on Mac:
Log in to https://www.gog.com on the web
Hover over your account name and choose Games
Find Stardew Valley
Click down arrow at right and click View Downloads
Under Extras, choose MAC 1.5.6 32bit Build
These aren't the only ways but what I have access to right now.
Steam on Windows:
Right click Stardew Valley and select Properties
Click Betas
Click the None popup next to Beta Participation
Choose legacy_1.5.6
gog.com on Mac:
Log in to https://www.gog.com on the web
Hover over your account name and choose Games
Find Stardew Valley
Click down arrow at right and click View Downloads
Under Extras, choose MAC 1.5.6 32bit Build
These aren't the only ways but what I have access to right now.
FilthyGorilla
Local Legend
In addition to what Lew said, unfortunately for other platforms there really isn't an alternative beyond having just stopped updates altogether for the app before the update released, something that's only really possible to consider in hindsight