The Centre on Regional Democratic Governance is collecting data on regional parliaments, regional executives and the rules that govern the relationship between parliaments and executives in the regions of more than 20 European countries. The data will probably be released in 2024 but in case you are interested in a particular region or country you can contact me.
A limited data release on regional executive government is available for thirteen countries. The dataset provides details on the parties which form the executive at the regional level (1 = party was part of the regional executive) and the article and appendix contain important information on the institutions of regional executive government in these thirteen countries. Please consult the article and the appendix before you use the data. Please contact me in case you are interested in this data.