You should add the steps below for modifying the InstalledPlugins.txt file to the upgrade instructions https://github.com/nopSolutions/nopCommerce/blob/develop/upgradescripts/3.80-the%20next%20version/Readme.txt Readme.txt file.
[i]Combine "Fixed Rate Shipping" and "Shipping by weight" plugins into single one. Upgrade
instructions: do not forget to manually delete "/Plugins/Shipping.FixedRateShipping" and
"/Plugins/Shipping.ByWeight" directories (they are not used anymore) and replace the plugin
«Shipping.Fixed Rate» or «Shipping.ByWeight» by the plugin «Shipping.FixedOrByWeight» in
the App_Data/InstalledPlugins.txt file.
Also, you should also mention, or even better, update the shippingbyweightenabled setting appropriately. Currently the upgrade script defaults it to False, meaning use Fixed Rate.
IF NOT EXISTS (S.ELECT 1 FROM [Setting] WHERE [name] = N'fixedorbyweightsettings.shippingbyweightenabled')
I.NSERT [Setting] ([Name], [Value], [StoreId])
VALUES (N'fixedorbyweightsettings.shippingbyweightenabled', N'False', 0)
It should then UPDATE it if shippingsettings.activeshippingratecomputationmethodsystemnames LIKE "%Shipping.ByWeight%"