The Finnegård, a component of the core zone and seat of the Hanseatic Museum, is being "pulled up from the ground" where it has slowly sunk (so I have been told) and will stay closed for this major restoration until 2025 at least.

Several beautification projects are going on at the same time, causing monumental facades to be covered, the Calle Las Damas broken up for street repairs and the Ozama fort to be closed. (Feb 2024)

Manjanggul Lava Tube is closed until (at least) August 2025
See www.visitjeju.net

Aranjuez, Casa del Labrador (scaffolding all around on the outside & no visit of the inside possible)

Palace of Westminster - future restoration (that may take up to 6 years)
See www.restorationandrenewal.uk

The Forty Mile historic site is closed due to flood damage since May 2023
See www.yukon-news.com

Eastern wing of Augustusburg is currently under restoration (scaffolding). The outside is covered, but the inside is still open.

The site museum/interpretative center is closed for renovations "until further notice" (still valid July 2023)

The House of Wonders and the People’s Palace museum are both being renovated, plus road works.

Restoration work began in 2021 and is scheduled to be completed in 2025. During the restoration, the mosque will be open for worship and visit.
See www.dailysabah.com

The second tower is closed for restoration work since November 2021 (and was still closed in July 2023).
See www.visitsanmarino.com

The website of the town of Spoleto states "the Basilica of San Salvatore is currently not accessible. It is not possible to enter but it is still possible to admire the interior from the main door." No date is given when the Basilica should be visitable again.

Chora church (only the Narthices are open)

Holy Trinity Column in Olomouc is going to be covered in scaffolding until the end of 2026.

The ancient frescoes in the Gelati Monastery have been damaged by rainwater due to improper restoration of the roof, the UNESCO has claimed. The UNESCO has notified Georgian authorities that the roof should be repaired not later than February 2022, since its absence could lead to further damage to the temple. - the monastery is fully closed until August 15, 2024
See www.eng.kavkaz-uzel.eu

Ponte Vecchio - from Oct 2024 to 2026.
See edition.cnn.com

Church of Santa María del Naranco: closed from Oct 2023 for at least 4 months
See www.santamariadelnaranco.es

There is restoration work in progress in the Basilica of Santa Maria degli Angeli. The Transit Chapel is not accessible and cannot be visited. The works started in May 2023 and are projected to take 14 months. An information table in the church states everything should be finished by September 2024. (Website) Apart from that, the Santuary of Rivotorto is also closed for works. Their website doesn't state any end date.

Pergamon Museum: "closed to visitors starting October 23, 2023. The institution is not scheduled to partially reopen until spring 2027 – with some sections of the museum, including the famous Ishtar Gate, scheduled to stay closed until 2037."
See www.lonelyplanet.com