* lisp/org-persist.el (org-persist--get-index): Check index file.
(org-persist--read-index): Warn when index file is missing.
(org-persist-write): Report write access errors.
* lisp/org-compat.el (org-file-name-concat): Do not use
`string-empty-p'.
(combine-change-calls): Create a stub when `combine-change-calls' were
not yet available.
(org-replace-buffer-contents): Add compatibility function for
`replace-buffer-contents'.
* lisp/org-element.el (org-element--current-element): Do not use
`if-let'.
* lisp/org-persist.el (org-persist-gc): Do not use `when-let'.
* lisp/org-plot.el (org-plot/gnuplot): Do not use `if-let'.
* lisp/org-src.el (org-edit-src-save, org-edit-src-exit): Use
`org-replace-buffer-contents'.
* lisp/org.el (org-narrow-to-subtree, org--property-local-values,
org-entry-get-with-inheritance, org-in-commented-heading-p,
org-up-heading-safe, org-goto-first-child): Do not use
`if-let'/`when-let'.
* testing/org-test.el (org-test-at-time): Fallback to old
`decode-time' specification in older Emacs.
* lisp/org-compat.el: Remove `length>' alias
* lisp/org-element.el (org-element-cache-map): Suppress compiler
warning.
* lisp/org-persist.el (org-persist-register): Do not `push' into
`plist-get'. Not supported yet in Emacs 27.
(org-persist-unregister): Do not use `length>'. Use
`org-directory-empty-p'.
(org-persist-gc): Use `org-directory-empty-p'.
Ihor Radchenko