If I remember correctly, best practice on a drawing is if you fully dimension a part in two out of three orthographic views you don’t include a third view with no dimensions. Is this a documented standard, best practice, or not true at all? An example would be a sheet metal rectangle with a hole. The front and side views would include all the necessary dimensions, leaving a top view with no dimensions. Should that top view be included?
My understanding is that redundancy and/or views that are not repeated should not be present in the a drawing. To put it another way: include only the necessary bits to have a complete picture.
So, dimensions that can be derived from other dimensions are discouraged. Having said that there are exceptions where some redundant dimensions/views might be useful and they are included in the drawing.
I can't really comment to whether its in a standard (there are a few), but its generally considered good practice and encouraged.
Regarding your example with the sheet metal, in one of the two orthonormal view the thickness of the sheet can be seen in one of the views. Also, in most cases you should be able to provide the hole depths in a well placed section view. So normally you don't need all three view.
In my experience, when giving out drawing for plasma/laser/hydro cutting of metal sheets only one view is provided (because through holes are assumed and the thickness is provided in the legend of the drawing).
Regarding the issue of repeat dimensions, at least for machine drawings as opposed to structural, reference dimensions (either in parentheses, or indicated like "12.34 REF") could be an option, to clarify that it shouldn't be interpreted e.g. with the default tolerance based on number of digits... both ASME Y14.5 and the counterpart ISO standards have this meaning for parentheses.
Regarding undimensioned orthographic views - For a "2D" part, including any sheetmetal without bends, I would say no. More generally, use your judgment case by case. Does it clarify, or confuse? Does having the extra view help relate the projected views in a more intuitive way? Where I work the mech designer always shows a (typically undimensioned) isometric view, so that might take away the need.
For your example:
The top view of the sheet metal (with "t" stated) will show all necessary information for fabrication. So, it can be a complete drawing by itself for documentation purposes. But, you need two side views to deliver the complete messages, with or without the top view.
There are controversies on whether no-redundant dimensioning or dimension-it-all is the better method. My personal view is it depends on the complexity of the work. It seems adequate and better for the drawing of mechanical parts not to provide the redundant dimensions to prevent conflicts, especially when changes require re-dimensioning. However, for a civil-structural or architectural project that contains numerous drawings, it is sometimes can be considered unfriendly to the users, who have to browse through multiple drawings just to find the information.
So, is there a standard industrial-wide drafting standard/rule? I don't think so. If in doubt, check with your boss, as it may vary from company to company, or job to job.
I think clarity is more important than following an arbitrary standard.
If the part is complicated, it may help to include a shaded 3d view. If it's not dimensioned, and everything is generated from a cad program (eliminating the possibility of inconsistencies between the views), then it can't hurt. You can even add a label: "for reference only, refer to orthographic views for dimensions"
At big companies the lawyers may get involved leading to rules for purpose of accountability, but in the absence of that your goal is to create a drawing which is easy to read, and which results in a part made consistently and correctly, that's what matters.
There is no reason why you can not add more views. But you should take care to minimize confusion. Adding a nondimensioned view can be neccesery to clarify some things.
It can be useful but you also have limited realestate on the sheet, more can be harder to read. Each projection requires the recipient to think more unless its clear they can skip the image. You also dont want to overwhelm the recipient with dimensions.
Remember that the technical drawing is often interpretted as a contractual obligation. For example in mechanical engineering typically your dimensioning scheme is interpretted as acceptance criteria. Therefore you dont want to over dimension as this might cause a conflict in your tolerance stackup. This could mean you need to accept parts that at worst are unusable for your purpose. Therefore extra dimensions should be made reference only.
The question of extra views is most probably not a problem if you are using a 3D application to generate them. The rule probably comes from cautioning that in hand drawing you might have a mistake that causes misinterpretation. The whole purpose of technical drawings is to disambiguate so you should avoid situations where you could cause issues. So it may not be worth the trouble.