When will the actual parish register of St Marys Steps, Exeter, Devon, England be available to study?
I assume you already realise the Bishop's Transcripts for the parish are available. (See https://www.familysearch.org/search/catalog/1832139?availability=Family%20History%20Library)
As far as the original parish registers are concerned, the "Catalog" page (at https://www.familysearch.org/search/catalog/2828535?availability=Family%20History%20Library) doesn't seem to be of much help in respect of their current availability. There are no DGS reference numbers provided, which would suggest the films were digitized after the Catalog was locked, about two years ago.
The location of the films is shown as Granite Mountain Record Vault. I don't know if this means they are completely locked to public access, or if they can be ordered for viewing at Salt Lake City.
As far as future access is concerned, I doubt you will be able to receive a direct answer. FamilySearch is continuously negotiating with record custodians to enable access to their material through the FamilySearch website, but never provide information regarding future accessibility of specific material.
There is material available for Devon Parish Registers at https://www.familysearch.org/search/image/index?owc=https://www.familysearch.org/service/cds/recapi/collections/1804330/waypoints, but this lists only film numbers - none of which do not appear to relate to St Mary Steps.
I have not yet checked if Ancestry, Find My Past, or another commercial concerns have the images on their websites. If so, they can probably be viewed for free at a Family History Centre, Affiliate Library, or even at your own local library - assuming you do not subscribe to these websites.
The images appear to be online at FindMyPast - subscriptions apply. When I checked them, they do not appear to be FamilySearch films - they are in colour with a distinctive multi-coloured header that includes "Images Courtesy of South West Heritage Trust and Parochial Church Council". This header appears on many Devon "films" that I've seen on FMP and elsewhere.
My best guess, therefore, is that they can only be copied and released with the agreement of the South West Heritage Trust (the Record Office) and the Parochial Church Council concerned. There must be every chance that FMP negotiated a period of exclusive access for digitising those films - standard practice in the UK. Even after that period is up, FS would need to either talk to the owners of the "films" or photograph them again themselves.
If all this sounds a bit vague, that's because it is. FS seem to have enough problems tracking their own films without looking elsewhere at potential research.
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