This can be already done in few ways, depending on what advanced functionality do you want to strip/preserve/add. So IMHO, this request can be archived.
(a) In OS, Set preferred default app for opening/editing “.md” files. Then in Obsidian use command/hotkey “Open in default app”.
(b) Use theme, e.g. Theme: Simple Plaintext
(c) Use CSS snippet which modifies/preserves only features that you want.
For example the following shows headers in standard font-size and removes strike-through line but preserves bold text, elevated block-IDs and footnote references, …
:root {
--font-size: 100%;
--font-family: Source Code Pro, Fira Code, monospace;
}
.markdown-source-view {
font-size: var(--font-size);
font-family: var(--font-family)
}
.cm-header {
font-size: var(--font-size);
font-family: (--font-family);
}
.cm-strikethrough {
text-decoration: none
}
Related:
Whitespace characters can be shown by Show whitespace plugin .