Suppose I own a patent that has not expired. How do I legally and reliably transfer it to the public domain?
The patent owner writes, and properly signs, a letter disclaiming all of the claims and files it in the case with the USPTO.
See MPEP section 1490 Disclaimers [R-08.2017]
35 U.S.C. 253 Disclaimer.
(a) IN GENERAL.—Whenever a claim of a patent is invalid the remaining claims shall not thereby be rendered invalid. A patentee, whether of the whole or any sectional interest therein, may, on payment of the fee required by law, make disclaimer of any complete claim, stating therein the extent of his interest in such patent. Such disclaimer shall be in writing and recorded in the Patent and Trademark Office, and it shall thereafter be considered as part of the original patent to the extent of the interest possessed by the disclaimant and by those claiming under him.
(b) ADDITIONAL DISCLAIMER OR DEDICATION.—In the manner set forth in subsection (a), any patentee or applicant may disclaim or dedicate to the public the entire term, or any terminal part of the term, of the patent granted or to be granted.
The EPO also has a process for this although they also indicate that one could follow procedures in any or all locations where it has been validated according to whatever process a country might have. Revoking or surrendering with the EPO kills it everywhere. NOTE - this is not called "disclaiming" at the EPO, that term is used for what is also termed "negative claiming".
1.2.5 Revocation of the patent in the event that the patent proprietor no longer wishes the patent to be maintained as granted
If the patent proprietor states that he no longer approves the text in which the patent was granted and does not submit an amended text, the patent must be revoked pursuant to Art. 101 (see T 203/14 and T 2405/12). This also applies when the patent proprietor requests the patent to be revoked.
If a patent proprietor unambiguously declares to the EPO the surrender (or abandonment or renunciation) of the patent, this is interpreted as equivalent to a request that the patent be revoked (see T 237/86). If the request of the patent proprietor is not unambiguous, he is given the opportunity to request that the patent be revoked or to declare that he no longer approves of the patent being maintained as granted. This results in the patent being revoked
I really know nothing about this. However after a bit of searching I did find the concept of "Patentleft" which seems to be an open source license for patents. Here is the Wikipedia page which is unfortunately brief. I also found the Open Invention Network which seems to be targeting patents related to Linux specifically. Lastly, this paper seems particularly relevant but is too long to summarize here.
It is possible to file paperwork with the patent office to disclaim all or some claims of a patent. I do not think there is a form for this. IBM famously disclaimed US6329919B1 System and method for providing reservations for restroom use. I would link to a copy of the disclaimer they filed but the application is old enough that the image file wrapper is not on line in PUBLIC PAIR.