Although there is no hard and fast rule, best practice is considered to be 12pt or 16px and it is agreed you should not go under 9pt or 12px. You should also consider that visual font size is not always represented in pt or px. Some fonts will look smaller than others at 12pt. Additionally, decorative fonts reduce legibility, so use with care.
WCAG grading with double and triple A contrast ratings are the current standard. You can check contrast ratings here as well as examples of would be seen by people with different visual impairments.
Clearly visible elements as well as the use of standard design principles, increases accessibility. These include contrast and size, balancing visual content and a clear hierarchy of importance that prioritises key information.
When using images, videos, illustrations and graphics, there are technical accessibility considerations. For instance, ensuring captioning is available or added on all video content.
However, broader contextual considerations are important to examine, eg. who or what is represented and how. Are you aligning with certain stereotypes? If so, what and why? Will the visuals represent the same thing to their audience that they represent to you? What might impact how someone will interpret them?
Many accessibility considerations are specific to the digital world, more info here. Digital spaces are often able to adopt technological advances faster, AI products like UserWay or AccessiBe allow people to personalise their experience on a website to fit their specific needs.
Capitalising words in hashtags is easier to read and enables screen readers to decipher. Emojis have alternative text inbuilt, so a long string of emojis will be individually read by most screen readers.
In order to ensure PDF documents are accessible a reading order must be set. This often requires a combination of adjustments in the program used to design the document and specific setup in Adobe Acrobat. You can find more info here and here. Changes are required to make most PowerPoint and Word documents accessible, see Web Aim course.
In addition to broadly applicable design considerations, (font size and legibility, colour contrast, layout and hierarchy and selecting and using visuals), it is important to ensure that alternative formats are on offer for your print collateral eg. digital documents, audio or braille.