This is a genealogically significant historical document.
A genealogically significant historical document is one that contains information about an individual or a family. A genealogist is a person who searches historical documents for information about individuals and families and then analyzes, organizes, and records the information. A genealogist should identify the historical document using a citation. A citation is a formal, consistent way of identifying the document or record so that other genealogists can find the record or document. The genealogist may also explain the thought process used to come to a conclusion about the reliability of the historical document or record. Once a document or record has been processed by a genealogist, we refer to that document or record as a source of the genealogist's opinion about the accuracy of the genealogist's compiled family history.
Here is the citation to the document above:
Shelden, Henry. 1870. Exemplified copy of last will and testament of Henry Shelden: late of the city of Brooklyn, deceased, and of the probate thereof, made under act of Congress of the 26th, May, 1780. https://www.worldcat.org/title/exemplified-copy-of-last-will-and-testament-of-henry-shelden-late-of-the-city-of-brooklyn-deceased-and-of-the-probate-thereof-made-under-act-of-congress-of-the-26th-may-1780/oclc/936575375&referer=brief_results, viewed 4 May 2022.
Finding these genealogically significant documents, analyzing the information contained in the documents and then recording that information in way that can enhance the abilities of others to also view the documents in context, are all part of the core work of being a genealogist. Accuracy and persistence are absolutely essential to making any progress as a genealogist.
When any entry lacks a source, this immediately implies inaccuracy and unreliability. But what about personal knowledge or oral history? Both of these are valid sources, and they still require a citation so someone reviewing the entries can tell where the information came from.
What about books? Well, if the author has footnotes or the equivalent and identifies the historical source, then you should record both the citation and the book. However, it the author simply lists all the information without further source citations, you have to discount the accuracy and if you decided to include any of the information you would cite the book as the source.
If you want a complete explanation of citations, you can refer to the following book.
Mills, Elizabeth Shown. 2007. Evidence Explained: Citing History Sources from Artifacts to Cyberspace. Baltimore, Md.: Genealogical Pub. Co.
You don't have to worry about genealogy books going out of date unless they are written about some technological subject such as online resources or DNA.
Of course, discovering genealogically relevant documents and records is the initial and most time consuming part of genealogical research and it is obvious, once the records or documents are finally discovered, they need to be searched. By the way, it is unwise to rely solely upon online resources. Granted, there is a lot of information online, but there is still a huge number of records waiting to be digitized, indexed, and/or cataloged.
To summarize, if you add any information to the FamilySearch Family Tree (excluding correcting existent information) you need to add a source with a citation to where that source can be found.