The base Store Locator Plus plugin and add ons are updated on a regular basis. Often the updates include security and bug fixes or fundamental changes that are designed to improve performance by reducing the memory and/or disk request load when your visitors are interacting with the map.
The most common reason your legacy add on stopped working is because it is no longer supported with the latest version of Store Locator Plus or vice versa
You may wish to choose to stay with your WPSLP Self Managed plugin and upgrade to the replacement bundled add-ons or switch to the managed MySLP.
Make sure you have a backup copy of the current version of SLP you have installed that IS working with your add on packs. We ALWAYS recommend that your site uses VaultPress or JetPack
Whenever you update the base plugin make sure ALL of your SLP add-on packs are updated to the latest version.