The concept of open government data involves the generation of data by governments and its availability in an open format for reuse. To be truly valuable in a smart city context, these data need to be tailored to the specific needs of cities. This longitudinal study aims to compare data openness and availability on four Portuguese municipal council websites between 2014 and 2024. It builds upon a 2015 study documenting open data availability, using the Linked Open Data principles and levels of certainty for characterization. The evaluation is based on the five open-data levels proposed by Tim Berners-Lee, and the attributes and value coding are adapted from recommendations by the Open Knowledge Foundation, World Wide Web Consortium, and Dublin Core Metadata Initiative. Data collected in 2024 was analyzed to assess the evolution of open data availability. Results comparing data availability in 2014 and 2024 indicated Guimarães presented marked improvement, while Braga and Famalicão showed significant progress. However, the incomplete data availability highlighted areas for further development. The evolution of data availability in these cities reveals a substantial commitment to improving data transparency and accessibility. Barcelo’s results demonstrated a lack of progress in data availability and updates. This stagnation impacts urban management, public safety, regulatory transparency, and citizen engagement. To achieve Guimarães’ commitment to data transparency and openness, Braga and Famalicão must close the gap related to publishing environmental open data and promote the United Nations 2030 Agenda’s Sustainable Development Goals focused on open data.