Is Ukraine constitutionally a neutral state?
The Ukrainian constitution[1] doesn't mention anything connected to a neutral state.
The only document that mention neutrality is "DECLARATION ON THE STATE SOVEREIGNTY OF UKRAINE"[2] on 1990 year:
The Ukrainian SSR ceremoniously proclaims its intention to become a permanently neutral state in the future, which will be out of military blocks and will be committed to three non-nuclear principles: not to accept, not to produce and not to acquire the nuclear weapon.
The document itself doesn't obligate Ukraine to be neutral, it only says that Ukraine had the intention to become a neutral state. And Ukraine was loyal to this idea even after Russian first attempt to annex Ukrainian territory(Tuzla Island conflict[3]) in 2003.
To make the neutrality status of Ukraine real, the Russia-friendly president of Ukraine Victor Yanukovich with the help of his part, who was a major in the Ukrainian parliament in 2010, voted for the non-block bill[4].
Only Russian illegal invasion of Ukraine and annexation of Ukrainian Crimea in 2014[5] pushed Ukraine to admit that neutrality will not guarantee safety for Ukraine - only strong partners will. After the Revolution of Dignity, the Ukrainian parliament removed newborn non-bloc law from the constitution[6]. In 2019 Ukraine's parliament voted on changes to Ukraine's constitution to confirm the path into NATO and EU[7]. As a sovereign country, Ukraine has every right to decide on its foreign affairs that do not contradict its constitution.
Since 2019, due to the constitutional change, this has also included participation in defense alliances.
1. Constitution of Ukraine
https://zakon.rada.gov.ua/laws/show/254%D0%BA/96-%D0%B2%D1%80?lang=en#Text
2. Declaration on the state sovereignity of Ukraine
https://zakon.rada.gov.ua/laws/show/en/55-12
3. Tuzla Island conflict
https://www.ukrainianworldcongress.org/20-years-since-russian-ukrainian-tuzla-island-conflict/
4. Ukraine's parliament votes to abandon Nato ambitions
https://www.bbc.com/news/10229626
5. Vote by UN General Assembly Isolates Russia
https://www.nytimes.com/2014/03/28/world/europe/General-Assembly-Vote-on-Crimea.html
6. Ukraine votes to drop non-aligned status
https://www.bbc.com/news/world-europe-30587924