Decidim
Decidim is a Participatory Democracy Platform that is used in over 30 municipalities. It is often mentioned in the context of participatory budgeting and is closely related to initiatives like PoliPoli and Call4. If you want to learn more about what Decidim is and its ideals, you can refer to the What is Decidim? - Ideals, etc. page on Notion(https://www.notion.so/Decidim-2afb014f33e6442aae46c32d375d29ef) or the What is Decidim - Google Slides presentation.
Code for Japan offers a SaaS service for local governments and their operations around security can be a valuable reference. You can find more information about Decidim security in the Supplementary: Decidim Security page on Notion. Additionally, there is a Japanese version of Decidim available in the Code for Japan Decidim repository(https://github.com/codeforjapan/decidim-cfj). The repository also contains development documentation.
If you want to use Decidim, you have the option to use the hosting service provided by Code for Japan as a Multi-tenant. Code for Japan's Decidim is a SaaS service available at the URL ＜subdomain＞.diycitie.jp. As of April 2021, there is a monthly fixed fee based on usage scale, but in the future, it is planned to be provided as one of the services of the "DIY City Project". You can find more details in the Decidim(https://www.notion.so/Decidim-877d2dbd578d4b4a81c26eb263f63ae2) page on Notion.
For official Decidim documentation, you can refer to the Welcome to Decidim Documentation :: Decidim Docs page. There is also an admin manual available in PDF format: Decidim_AdminManual_EN_0.10.pdf.
If you're interested in the perspective of CivicTech and hal sk on Decidim, you can visit Decidim - Hal Seki's Scrapbox.
In a note(https://note.com/kken78/n/nd828c4b7e82c#ujUYL) by HIGASHI Kenjiro, it is recommended to read a book about Decidim as it covers most of the current initiatives and provides insights on why the boom has faded. You can find more information in a tweet by heykuro1 on September 16, 2021(https://twitter.com/heykuro1/status/1438370436819996672?ref_src=twsrc%5Etfw).